Support ALPN for TCP + TLS routers

[dsseng]

What does this PR do?

Support matching ALPN protocols when initiating connection with a TLS client, thus allowing to multiplex different service behind a single TLS entrypoint (e.g. handling h2 in Traefik and connecting mqtt to an MQTT broker)

Motivation

fixes #7906

More

• Added/updated tests
• Added/updated documentation

Additional Notes

Tested using client code from https://github.com/jefferai/golang-alpn-example/blob/master/alpnexample.go as foo protocol (On Go 1.16) and Chrome M102 as h2 client

[dsseng]

This draft can be reviewed, code does the expected things, but docs and probably tests are left to be done

[dsseng]

Validate broke due to "misspell" triggering by mosquitto and Semaphore tests do not seem to be related to the topic of changes.

[rtribotte]

Hello @sh7dm,

Validate broke due to "misspell" triggering by mosquitto and Semaphore tests do not seem to be related to the topic of changes.

No worries, we will take a look at the unit tests during the review phase.
We will come back to you once the design review iteration is done.
Just to let you know, as of now, we are wondering about the interactions when using this matcher with ACME challenges enabled.

[dsseng]

I have tested that, looks like nothing broke, but it should be reviewed to ensure it fits great and does not induce a risk of regressions. Thanks for informing!

[rtribotte]

Thanks 👍

[tomMoulard]

LGTM 👌

[mpl]

Looking at the code again, I'm confused by some aspects of the (existing) behaviour of clientHelloInfo, but double-checking that is out of the scope of this PR. Will do later.