WIP Add ability to use external plugins for project services

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.
• Add plugins to list of available services on project integrations page.

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

Why was this MR needed?

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
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

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