Activity feed webhooks backend

[getvictor]

#14722 Added gitops/apply configuration support:

  webhook_settings:
    activities_webhook:
      enable_activities_webhook: true
      destination_url: "https://example.com/activity"

If the webhook response is a 429 error code, the webhook retries for up to 30 minutes.

Video commentary: https://www.loom.com/share/c20a06c7b7614d379353ae47c2594237

Checklist for submitter

• [x] Changes file added for user-visible changes in changes/, orbit/changes/ or ee/fleetd-chrome/changes. See Changes files for more information.
• [x] Added/updated tests
• [x] Manual QA for all new/changed functionality

[getvictor]

@mostlikelee Please re-review. I made a couple of small fixes.