[JENKINS-75144] Add async processing for webhook events in Organizati…

[sinedd777]

…onFolder

Fixes the issue where one unauthenticated OrganizationFolder hitting GitHub API rate limits would block
webhook processing for all other OrganizationFolders.

Changes

• Created OrganizationFolderAsyncHelper to handle conditional async processing
• Modified onSCMHeadEvent, onSCMNavigatorEvent, and onSCMSourceEvent to use async processing when
  multiple folders exist
• Uses CompletableFuture with timeout to prevent indefinite blocking
• Maintains synchronous behavior for single folder scenarios (for compatibility)

The fix ensures that authenticated folders can continue processing webhooks even when unauthenticated
folders are rate-limited by GitHub's 60 requests/hour limit for anonymous access.

Testing done

• Set up multiple OrganizationFolder instances with mixed authentication (some with GitHub credentials,
  some without)
• Triggered webhook events to exhaust the 60 requests/hour rate limit on unauthenticated folders
• Verified that authenticated folders continue processing webhooks without being blocked
• Confirmed async processing activates only when multiple folders exist
• Tested timeout behavior to ensure no indefinite blocking occurs
• Verified backward compatibility by testing with a single OrganizationFolder (remains synchronous)

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main
  branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira -
  JENKINS-75144
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed