Add smart Rate Limiter

[seratch]

Now that we have a built-in retry handler for rate limited errors in v3.9, retrying a request after getting rate-limited is much easier than before.

• https://github.com/slackapi/python-slack-sdk/releases/tag/v3.9.0
• https://github.com/slackapi/python-slack-sdk/blob/v3.9.1/slack_sdk/http_retry/builtin_handlers.py#L52-L91
• https://github.com/slackapi/python-slack-sdk/blob/v3.9.1/slack_sdk/http_retry/builtin_async_handlers.py#L49-L88

However, getting rate-limited results in a much longer execution time as the whole operation. As a solution for this pain point, we will add a built-in traffic control layer like we do in the Java SDK: https://slack.dev/java-slack-sdk/guides/web-api-basics#rate-limits

A few points to consider for implementing this feature:

• The module manages the amount of requests in the last minute and calculate the interval before the next API call
• Metrics data can be in memory in most use cases but the part should be extensible to switch to other data storage (e.g., Redis)
• As we do in the Java SDK, running the same script/app in multiple hosts should be supported.
• For backward-compatibility, we don't turn the feature on by default. Users can enable it only when they want it.
• Both sync and async API clients should be supported in a proper way (= don't use time.sleep in asyncio apps)
• All API clients (Web API, Audit Logs, SCIM, Webhook) should have the functionality.

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.models (UI component builders)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.socket_mode (Socket Mode client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.scim (SCIM API client)
• slack_sdk.rtm (RTM client)
• slack_sdk.signature (Request Signature Verifier)

Requirements

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[seratch]

I've implemented a similar thing in the Discovery API SDK: https://github.com/slackapi/python-slack-discovery-sdk/blob/main/slack_discovery_sdk/rate_limit_support.py The ones for the APIs supported by this SDK (especially the Web APIs) would require a bit more complex logic, though.