implement H2C to proxy to h2 over TCP

[szuecs]

Is your feature request related to a problem? Please describe.

For example in Kubernetes, people want to use server push, etc. features from h2.

• server/client ALPN/Upgrade h2/h2c: https://github.com/nekolunar/http2
• very short unmaintained: https://github.com/owlwalks/unload/blob/master/main.go
• h2i console https://godoc.org/golang.org/x/net/http2/h2i
• h2c handler https://godoc.org/golang.org/x/net/http2/h2c#ex-NewHandler
• h2 configure server https://godoc.org/golang.org/x/net/http2#ConfigureServer

Describe the solution you would like

The proxy should be able to connect with H2C.

Describe alternatives you've considered (optional)

Create mTLS infrastructure, which can be a huge change without providing too much value

Would you like to work on it?
Yes

[AlexanderYastrebov]

Related #1868 enables incoming h2c connections