Runtime Contract: Update Protocols and Ports section

[dgerd]

• Describe current behavior for HTTP1.1 and HTTP2
• Provide links to relevant tests for keywords

Ref: #4035
Fixes: #4283

Release Note

Update Protocols and Ports section of the runtime contract.

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[evankanderson]

The automatic upgrade would be that the queue-proxy would include the Upgrade: h2c header. The client could continue to return HTTP1/.1 results.

https://tools.ietf.org/html/rfc7540#section-3.2

   A server that does not support HTTP/2 can respond to the request as
   though the Upgrade header field were absent:

If the container wants to support HTTP/2, they can respond with a 101 Switching Protocols and then send the response by HTTP/2 if it wants to.

   A server that supports HTTP/2 accepts the upgrade with a 101
   (Switching Protocols) response.  After the empty line that terminates
   the 101 response, the server can begin sending HTTP/2 frames.  These
   frames MUST include a response to the request that initiated the
   upgrade.

We might be able to handle this in the queue-proxy, rather than needing to do it in Envoy (e.g. always HTTP/2 to the queue-proxy and then convert it there).

[markusthoemmes]

@dgerd this seems to need a rebase.

/assign @evankanderson