reverseproxy: Add unix+h2c Caddyfile network shortcut

[francislavoie]

As requested in #4498, just adding a little shorthand to make this work:

reverse_proxy unix+h2c//dev/shm/grpc

/cc @PaTTeeL

[mholt]

I do find it strange that h2c is set as a scheme and that unix+h2c is a network type, but it kinda makes sense. I can't think of a better alternate way to do it, so I guess we can go with this.

[francislavoie]

Yeah, that's just kinda how the code was set up. We have the "Caddy network addresses" which map to Go's stuff, and then we have the scheme shortcuts for Caddyfile upstream addresses... and H2C + Unix is actually both of those.

I'm using + cause we have precedent with srv+https (although that's deprecated now with dynamic srv but I digress)