Add built in server error retry handlers

[digitalnomd]

Summary

This pull request resolves #1364

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

• slack_sdk.web.WebClient (sync/async) (Web API client)
• slack_sdk.webhook.WebhookClient (sync/async) (Incoming Webhook, response_url sender)
• slack_sdk.socket_mode (Socket Mode client)
• slack_sdk.signature (Request Signature Verifier)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.models (UI component builders)
• slack_sdk.scim (SCIM API client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.rtm_v2 (RTM client)
• /docs-src (Documents, have you run ./scripts/docs.sh?)
• /docs-src-v2 (Documents, have you run ./scripts/docs-v2.sh?)
• /tutorial (PythOnBoardingBot tutorial)
• tests/integration_tests (Automated tests for this library)

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run python3 -m venv .venv && source .venv/bin/activate && ./scripts/run_validation.sh after making the changes.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @digitalnomd to sign the Salesforce Inc. Contributor License Agreement.

[seratch]

We already have a few unit tests for the built-in retry handlers. If you could add some for the new handler, it'd be greatly appreciated. Otherwise, I can add some tests when merging your changes.

• https://github.com/slackapi/python-slack-sdk/blob/main/tests/slack_sdk/web/test_web_client_http_retry_server_error.py
• https://github.com/slackapi/python-slack-sdk/blob/main/tests/slack_sdk_async/web/test_async_web_client_http_retry.py

[codecov]

Codecov Report

Merging #1367 (485a33e) into main (7eec685) will decrease coverage by 0.04%.
The diff coverage is 60.00%.

❗ Current head 485a33e differs from pull request most recent head 3738060. Consider uploading reports for the commit 3738060 to get more accurate results

@@            Coverage Diff             @@
##             main    #1367      +/-   ##
==========================================
- Coverage   85.58%   85.55%   -0.04%     
==========================================
  Files         111      111              
  Lines       11559    11569      +10     
==========================================
+ Hits         9893     9898       +5     
- Misses       1666     1671       +5     

Impacted Files	Coverage Δ
slack_sdk/http_retry/builtin_async_handlers.py	91.30% <60.00%> (-3.82%)	⬇️
slack_sdk/http_retry/builtin_handlers.py	91.48% <60.00%> (-3.75%)	⬇️

... and 1 file with indirect coverage changes

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[seratch]

Thanks for taking your time to make this PR! After merging the changes, I will add a few unit tests for them