Add ability to use external plugins as system hooks

What does this MR do?

Introduces support for local plugins:

• Added rake plugins:generate task that creates a plugin for you from the template.
• Validate plugins on application startup via config/initializers.
• Execute plugins in same events as system hooks.

Are there points in the code the reviewer needs to double check?

no

Why was this MR needed?

So GitLab administrators can introduce custom integrations without modifying GitLab source code.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/40812