Add BulkImportsController for user to provide credentials

What does this MR do?

This MR is part of &4374 (closed) epic to add new Group Import V2 that does not use archive files, but uses API instead.

It adds new BulkImportsController backend changes to allow user to provide authentication credentials with another GitLab instance. This MR does not include the actual authentication. The fetching of data with provided credentials will be done as part of another issue/MR (!43155 (merged))

Behind a feature flag bulk_import. Does not contain any frontend changes.

Demo of changes made (used with dummy frontend that can be found here: https://gitlab.com/-/snippets/2018180)

Mentions #249152 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content