BE Improve the GitHub and Gitea import feature table interface

What does this MR do?

This is the backend to vue-ify the import feature for github and gitea.

Please view the Table of MRs.

This MR specifically:

• Adds etag cache polling for realtime_changes, expire on status and create actions.
• Adds a number of serializers (should be a different branch, oh well)
• Attempts to simplify controller actions

Follow up MRs:

• Remove provider api call from Import:: GithubController#status and handle on clientside https://gitlab.com/gitlab-org/gitlab-ce/issues/55585

Table of MRs

	CE	EE
FE	https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24608	https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9295
BE	https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24606	https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9294

• ── CE master
   └── CE BE
       └── CE FE
• ── EE master
    └── EE BE
        └── EE FE

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer