Migrate API endpoints to FastAPI pt.1

[lbarcziova]

• migrate bodhi_updates.py, copr_builds.py, koj_builds.py, koji_tag_requests.py (here)
• update any request validation and response models using Pydantic
• keep old Flask endpoints functional

[Gauravpadam]

I've opened up a PR for this. Before making any more commits, Could I please get an initial direction on the PR @lbarcziova?
Thanks!

[lbarcziova]

@Gauravpadam hello! I will have a look, but unfortunately, as stated in #2745 (comment), we will not be able to merge this at this moment as we put this work on hold for now.