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

[Kludex]

I don't see much literature about receiving a comma-separated list. The main references I could find have either "https" or "http".

[vanschelven]

Do you not consider the list in the linked resource sufficiently authoritative enough or did you perhaps miss it? Quoted below:

Similar issues have been raised and implemented in other projects, for example:

Tornado:

• Issue: ​https://github.com/tornadoweb/tornado/issues/2161 https://github.com/tornadoweb/tornado/issues/2161
• Implementation: ​https://github.com/tornadoweb/tornado/blob/00c9e0ae31a5a0d12e09109fb77ffe391bfe1131/tornado/httpserver.py#L347-L350 https://github.com/tornadoweb/tornado/blob/00c9e0ae31a5a0d12e09109fb77ffe391bfe1131/tornado/httpserver.py#L347-L350

Ruby:

• Issue: ​https://bugs.ruby-lang.org/issues/10789 https://bugs.ruby-lang.org/issues/10789
• Implemenation: ​https://github.com/ruby/ruby/blob/d92f09a5eea009fa28cd046e9d0eb698e3d94c5c/tool/lib/webrick/httprequest.rb#L614-L616 https://github.com/ruby/ruby/blob/d92f09a5eea009fa28cd046e9d0eb698e3d94c5c/tool/lib/webrick/httprequest.rb#L614-L616

Reactor-Netty:

• ​https://github.com/reactor/reactor-netty/issues/976 https://github.com/reactor/reactor-netty/issues/976

• Implementation: ​https://github.com/reactor/reactor-netty/commit/e190d5bbf65d88d3a0240cd60b81e1ee1907030e https://github.com/reactor/reactor-netty/commit/e190d5bbf65d88d3a0240cd60b81e1ee1907030e

  [..]

Common example of this behavior is when using mulitple AWS proxies such as API Gateway proxying to an elastic load balancer.