Implement /internal/post_receive unified endpoint for PostReceive tasks

What does this MR do?

Implement /internal/post_receive API endpoint for all PostReceive tasks

Advances https://gitlab.com/gitlab-org/gitaly/issues/520

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

No

Why was this MR needed?

We want to get rid of Gitlab-Shell <-> Redis interaction to decouple the file storage nodes from Redis. See https://gitlab.com/gitlab-org/gitaly/issues/467.

By unifying all post-receive API operations, we also save round-trips between shell and rails, which can help with the issues discussed in https://gitlab.com/gitlab-org/gitlab-shell/issues/97

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

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitaly/issues/511