text: Title: Add retry handler for 500s to WebClient by default
  Body: Include custom retry handler for Slack 500s by default similar to how `ConnectionErrorRetryHandler` is included by default in the `WebClient`. I think this would be a good addition because as a developer using the slack sdk I'd expect this to be the default behavior to prevent transient Slack server errors from raising a `SlackApiError` 😄.
  
  ### Category (place an `x` in each of the `[ ]`)
  
  - [x] **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
  
  Please read the [Contributing guidelines](https://github.com/slackapi/python-slack-sdk/blob/main/.github/contributing.md) and [Code of Conduct](https://slackhq.github.io/code-of-conduct) before creating this issue or pull request. By submitting, you are agreeing to those rules.
  