Support Upgrade: h2c for upstream clusters

*Title*: *Support Upgrade: h2c for upstream clusters*

*Description*:

I have an environment where I have a mixture of HTTP/1.1 (plaintext) and HTTP/2 upstreams. What I'd like to be able to do is configure Envoy in such a way that it can automatically negotiate to use HTTP/1.1 or HTTP/2 without me manually specifying this. ALPN would be ideal here of course but nothing is currently using TLS so (as I understand things) that won't work.

Envoy already supports Upgrade: h2c for downstreams, would it be possible to get support for this for upstreams? I do understand that h2c is relatively abandonded/outdataed in favour of ALPN, but as I said that is not useable for me in my current circumstancs.

[optional *Relevant Links*:]

https://en.wikipedia.org/wiki/HTTP/1.1_Upgrade_header
https://github.com/envoyproxy/envoy/issues/7161