Better handling of cors_enable=True + added CORSMiddleware

[vytas7]

Although similar to #1930, this issue proposes to improve not only documentation, but also provide a clearer indication to the developer who has happened to pass this unsupported combination of parameters.

Maybe we could emit a warning, silently disable the implicitly added middleware component if we detect that any instance of CORSMiddleware is already present, or even raise an Exception? (Although the latter might be seen as a breaking change -- discuss).

[CaselIT]

connected to #1976

Probably the best choice here is to deprecate the flag, since any other case would not be immediately clear to understand.
An alternative may be to raise a deprecation warning and the raise an exception in the next major

[vytas7]

Hm, on the one hand, it may make sense to be explicit, on the other, it's convenient to have a flag to quickly enable permissive CORS, since it is what quite many users are after.

Maybe it wouldn't be too complicated to only prepend CORSMiddleware when updating the middleware list only if there is no other object where isinstance(component, CORSMiddleware)?

In either case, I have removed the contributor/Hacktoberfest labels before we make a decision on this, not to get any potential contributor dragged into this beforehand.

[vytas7]

See also #2095. Rescheduling for 4.0.

[vytas7]

I went ahead and decided to simply raise a ValueError in this case.

[CaselIT]

makes sense