Implement GitHub "on pull request" block

[Pwuts]

First webhook-triggered block implementation based on architecture sketch made in #8349.

Required parts

• Add DB tables for webhooks and their links to nodes
• Mechanism to register/update/delete webhook on block create/update/delete
  Tying this to the block's credentials instead of the block itself because the webhook has to be administrated using those credentials.
  • Block.on_node_update(cls, new_default_input, old_default_input = None)
  • DB table to store webhooks and their configs
• Endpoint to receive incoming webhook requests
  • one (parametrized) endpoint for all, with the possibility of making provider-specific exceptions if needed: /integrations/{provider}/webhooks/{webhook_id}/ingress
  • Logic to initiate a graph execution when a webhook request is received
    • Verify the legitimacy of the webhook request/payload
      • check in the receiving endpoint
    • Initiate a graph execution using the webhook payload
      • execution input = input_default + webhook payload
• On deletion of integration credentials -> delete all associated webhooks
• The block itself: GitHub On Pull Request (name subject to change)
  • inputs: credentials, repo (URL or {owner}/{repo_name}), on_event_types (create/update/delete etc.)
  • outputs: pull_request, event_type

Sub-issues

• Add DB tables for webhooks and their links to nodes #8354
• Implement mechanism to register/update/delete webhook on block create/update/delete #8353
• Endpoint /integrations/{provider}/webhooks/{webhook_id}/ingress #8356
• On deletion of integration credentials -> delete all associated webhooks first #8357