Add purge configuration and scheduled purger functionality

[fujiwara]

This PR adds a scheduled purge feature.

When a purge setting is available, mirage-ecs runs a purge task at the specified schedule. There is no need to call POST /api/purge from external.

---

purge section configures purge settings.

purge:
  schedule: "13 4 * * ? *" # cron expression
  request:
    duration: 86400
    excludes:
      - foo
      - bar
    exclude_tags:
      - "branch:preview"
    exclude_regexp: "^(foo|bar)"

The schedule is a cron expression to run the purge task.

• mirage-ecs runs the purge task at the specified schedule.
• The expression is the same as the Amazon EventBridge cron().
  • See the document of Cron expressions for details.
  • Timezone is depends on the ECS task timezone.

The request section is the same as the /api/purge API. See API Documents.