refactor: make GZipMiddleware excluded content types configurable

[shreyas-dev]

Summary

This PR refactors the GZipMiddleware to make excluded content types configurable via a constructor parameter instead of using a hardcoded constant.

Checklist

• [x] I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• [x] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• [x] I've updated the documentation accordingly.

[Kludex]

What is the issue you are having?

[petoknm]

The DEFAULT_EXCLUDED_CONTENT_TYPES variable is sometimes not what users want. In my case, I want event-stream to be compressed because I'm using data-star and returning a lot of very compressible html.

A workaround I've been using for now has been to either edit the variable directly in the package in my virtual enviroment or:

from fastapi.middleware.gzip import GZipMiddleware
from starlette.middleware import gzip

app = FastAPI()
app.add_middleware(GZipMiddleware, minimum_size=1000, compresslevel=5)
gzip.DEFAULT_EXCLUDED_CONTENT_TYPES = ()

which is not pretty IMO... would like this to be configurable the same way minimum_size and compresslevel are.