Consistently use the same OkHttpClient for files.slack.com requests

[seratch]

This pull request resolves a bug where the files.slack.com requests fail when a developer passes OkHttpClient to SlackHttpClient when initializing the API clients to handle specific SSL conditions. This pull request tweak the internal FilesUploadV2Helper class to use the given OkHttpClient rather than creating a new once only using SlackConfig properties.

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

All modified and coverable lines are covered by tests ✅

Project coverage is 73.25%. Comparing base (c8d803a) to head (3016cd5).
Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1428      +/-   ##
============================================
+ Coverage     73.21%   73.25%   +0.04%     
  Complexity     4377     4377              
============================================
  Files           474      474              
  Lines         14205    14206       +1     
  Branches       1437     1437              
============================================
+ Hits          10400    10407       +7     
+ Misses         2958     2951       -7     
- Partials        847      848       +1     

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