httpcaddyfile: Support compression in http transport config

[Vigilans]

I have successfully established h2c connection between caddy and my local service with the commits in #3561.

But the json config in #3556 (comment) cannot be simply ported to caddyfile because of the absence of compression field in reverse_proxy -> transport http config. This PR is just a minor modification to make this field available to caddyfile's http transport config.

Config example:

    transport http {
      versions h2c 2
      compression off
    }

[francislavoie]

For reference, the HTTP transport in JSON config has the compression field here: https://caddyserver.com/docs/json/apps/http/servers/errors/routes/handle/reverse_proxy/transport/http/

[mholt]

LGTM, thanks for the PR! I love these contributions that make simple and gradual improvements to the Caddyfile.

[mholt]

Feel free to contribute again sometime 👍