flutter/cocoon has several separate locations for scheduling. Unifying it all behind a single service will make it easier to track the scheduling logic Cocoon has.

Currently there are separate request handlers for scheduling initial tasks and retrying these tasks. Instead, there should be a scheduler service the request handlers call.

This will make it cleaner to schedule against webhooks while still supporting the cron methods.

Checklist

• Create scheduler service - [app_dart] Create scheduler service cocoon#1087
• Schedule prod tasks on GitHub webhook - [app_dart] Schedule prod tasks from GitHub webhooks cocoon#1109
• Recipes support checkouts from GitHub instead of GoB
• Validate .ci.yaml changes in presubmit - [scheduler] Create presubmit builder to validate configs #79579
• Support presubmit targets - [cocoon_scheduler] Support presubmit triggers #77858
• Prod LUCI tasks are updated - [scheduler] Update prod tasks listed in ci.yaml cocoon#1210
• Deprecate "validate_json" step: https://flutter.googlesource.com/recipes/+/refs/heads/master/recipe_modules/json_util/api.py