Add files.upload v2 support

[seratch]

This pull request adds a new way to upload files to Slack.

The legacy files.upload API endpoint now has severe performance issues as described in the following reports:

• File Upload API succeeds despite 408 Request Timeout python-slack-sdk#1165
• files.upload API return {'ok': False, 'error': 'error_creating_file_sync'} python-slack-sdk#1191

Slack platform team decided to unlock a new way to upload files using the following endpoints:

• Call https://api.slack.com/methods/files.getUploadURLExternal to fetch the URL to upload per file
• Use the URL https://files.slack.com/upload/v1/*** to upload the file content per file
• Call https://api.slack.com/methods/files.completeUploadExternal to finish the upload this time and share the files in a channel only once as the last step

This pull request adds supports for the new endpoints, so that now the low-level APIs are available for developers. Having said that, going through the above process for uploading files requires many lines of code on 3rd party app side. Also, following all the steps can be confusing for developers.

For this reason, in addition to the low-level API supports, I propose to add MethodsClient#filesUploadV2 method as a wrapper of the whole file-upload operation. Here are the code examples demonstrating how it works:

import com.slack.api.Slack;
import com.slack.api.methods.request.files.FilesUploadV2Request;
import com.slack.api.methods.response.files.FilesUploadResponse;
import com.slack.api.methods.response.files.FilesUploadV2Response;

import java.io.File;
import java.util.Arrays;
import java.util.List;

public class Sample {
  public static void main(String[] args) throws Exception {

    String token = System.getenv("SLACK_BOT_TOKEN");
    Slack slack = Slack.getInstance();

    // Legacy files.upload code example
    FilesUploadResponse legacyResponse = slack.methods(token).filesUpload(r -> r
        .file(new File("./logo.png"))
        .filename("logo.png")
        .title("Company Logo")
        .initialComment("Here is the latest version of our company logo!")
        .channels(Arrays.asList("C123456"))
    );
    com.slack.api.model.File legacyFile = legacyResponse.getFile();

    // V2 wrapper is mostly the same!
    FilesUploadV2Response v2Response = slack.methods(token).filesUploadV2(r -> r
        .file(new File("./logo.png"))
        .filename("logo.png")
        .title("Company Logo")
        .initialComment("Here is the latest version of our company logo!")
        .channel("C123456") // sharing a file in multiple channels is no longer supported
    );
    com.slack.api.model.File v2File = v2Response.getFile();

    // Now you can upload multiple files at a time!
    FilesUploadV2Response v2MultiFilesResponse = slack.methods(token).filesUploadV2(r -> r
        .fileUploads(Arrays.asList(
            FilesUploadV2Request.FileUpload.builder()
                .file(new File("./logo.png"))
                .filename("logo.png")
                .title("Company Logo")
                .build(),
            FilesUploadV2Request.FileUpload.builder()
                .file(new File("./2022-03-01-meeting-minutes.md"))
                .filename("2022-03-01-meeting-minutes.md")
                .title("2022-03-01 Meeting Minutes")
                .snippetType("text")
                .build()
        ))
        .initialComment("Here are the files I mentioned in the meeting :wave:")
        .channel("C123456")
    );
    List<com.slack.api.model.File> v2Files = v2MultiFilesResponse.getFiles();
  }
}

Once we agree that we can go with this design, I will add the same one to other SDKs (Node, Python).

Category (place an x in each of the [ ])

• bolt (Bolt for Java)
• bolt-{sub modules} (Bolt for Java - optional modules)
• slack-api-client (Slack API Clients)
• slack-api-model (Slack API Data Models)
• slack-api-*-kotlin-extension (Kotlin Extensions for Slack API Clients)
• slack-app-backend (The primitive layer of Bolt for Java)

Requirements

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you agree to those rules.

[codecov]

Codecov Report

Merging #1065 (30732b7) into main (612d9a7) will increase coverage by 0.07%.
The diff coverage is 81.34%.

❗ Current head 30732b7 differs from pull request most recent head 6265e15. Consider uploading reports for the commit 6265e15 to get more accurate results

@@             Coverage Diff              @@
##               main    #1065      +/-   ##
============================================
+ Coverage     76.80%   76.87%   +0.07%     
- Complexity     3733     3765      +32     
============================================
  Files           407      409       +2     
  Lines         11250    11384     +134     
  Branches       1128     1141      +13     
============================================
+ Hits           8640     8752     +112     
- Misses         1936     1948      +12     
- Partials        674      684      +10     

Impacted Files	Coverage Δ
.../com/slack/api/methods/impl/MethodsClientImpl.java	84.62% <73.33%> (-0.45%)	⬇️
...om/slack/api/methods/impl/FilesUploadV2Helper.java	80.48% <80.48%> (ø)
...java/com/slack/api/methods/RequestFormBuilder.java	88.43% <92.30%> (+0.03%)	⬆️
.../java/com/slack/api/methods/MethodsRateLimits.java	99.06% <100.00%> (+<0.01%)	⬆️
...slack/api/methods/SlackFilesUploadV2Exception.java	100.00% <100.00%> (ø)
...slack/api/methods/impl/AsyncMethodsClientImpl.java	92.95% <100.00%> (+0.09%)	⬆️
...imits/metrics/impl/BaseMemoryMetricsDatastore.java	80.08% <0.00%> (+0.43%)	⬆️
...va/com/slack/api/socket_mode/SocketModeClient.java	64.33% <0.00%> (+0.63%)	⬆️
.../java/com/slack/api/util/http/SlackHttpClient.java	87.68% <0.00%> (+0.72%)	⬆️

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[WilliamBergamin]

This looks good to me, slick work 💯