Add support for multiple values for the x-forwarded-proto header

[vanschelven]

See #2102

When uvicorn is deployed behind more than one proxy, the proxy behavior is sometimes to list the protocol as a comma-separated list.

However, currently, uvicorn expects only one value for the x-forwarded-proto header, instead of parsing it as a list of values and setting the protocol accordingly.

x-forwarded-proto is a non-standard header, so there isn't a specification for its use, but different reverse-proxy vendors do use it in different ways, and some append the protocol as a comma-separated value from left-to-right (left being the furthermost proxy and rightmost being the closest).

Checklist

• [x] I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• [ ] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• [ ] I've updated the documentation accordingly.
• [x] I'm a volunteer myself who's happy to provide a drive-by commit as a direction for a solution but cannot spend more time on this right now