OCT-1825: Initial passthrough FastAPI server (rebased)

[adam-gf]

Description

Adds FastAPI as the main application.
Mounts old flask app under /flask.
If route does not exist in FastAPI it's routed to old implementation under flask.
Rewrites socketio connection handling in FastAPI directly (using async).
Fixes the socketio manager to share sessions via redis.

Definition of Done

1. If required, the desciption of your change is added to the QA changelog
2. Acceptance criteria are met.
3. PR is manually tested before the merge by developer(s).
   • Happy path is manually checked.
4. PR is manually tested by QA when their assistance is required (1).
   • Octant Areas & Test Cases are checked for impact and updated if required (2).
5. Unit tests are added unless there is a reason to omit them.
6. Automated tests are added when required.
7. The code is merged.
8. Tech documentation is added / updated, reviewed and approved (including mandatory approval by a code owner, should such exist for changed files).
   • BE: Swagger documentation is updated.
9. When required by QA:
   • Deployed to the relevant environment.
   • Passed system tests.

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(1) Developer(s) in coordination with QA decide whether it's required. For small tickets introducing small changes QA assistance is most probably not required.

(2) Octant Areas & Test Cases.

[linear]

OCT-1825 Initial passthrough FastAPI server

[housekeeper-bot]

E2E job #11440458323 passed 🎉

[mslomnicki]

Approved as rebased #379