Allow setting of feature gates per project

What does this MR do?

Allows API calls to set feature gates for projects, as it would've been handy for: https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/5872

What are the relevant issue numbers?

None that I know of.

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• Documentation created/updated
• 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