Consider adding support for more web frameworks

While Flask is a decent choice as a default implementation, it may be beneficial to allow using different frameworks as the baseline for a Cloud Function.

I assume the following ones could be some great candidates:
* https://fastapi.tiangolo.com/
* https://www.tornadoweb.org/en/stable/
* https://sanic.readthedocs.io/en/latest/

We're also considering having framework-specific implementations of CloudEvents (e.g. FastAPI uses Pydantic by default and there's an open [PR](https://github.com/cloudevents/sdk-python/pull/182) for Pydantic support in CloudEvents).
This can probably be handled as "extras" for the main library.

Is it smth the maintainers may consider?