Convert Cadwyn to a middleware

[zmievsa]

Currently, FastAPI first validates a request in its correct version, and then we migrate request to latest. For responses, it goes in reverse: we migrate response from latest, and then FastAPI validates the response. This approach tightly binds our versioning logic to FastAPI and causes us to do a lot of hacks. Instead, we should use a middleware that will see each request first and convert it to latest. This will significantly simplify our code and make it easier for us to keep compatibility with future FastAPI versions.

1. Request comes
2. pick version (version picking middleware)
3. migration middleware
   1. Get body for old version
   2. solve_dependencies for old version
   3. convert to latest
   4. set api version header to latest version
4. run the endpoint

[zmievsa]

Alternatively, custom route classes might be a good fit.
https://fastapi.tiangolo.com/how-to/custom-request-and-route/