Caddy: support h2c protocol for reverse_proxy

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[x] I have read the contributing guide lines at https://github.com/opnsense/plugins/blob/master/CONTRIBUTING.md
[x] I have searched the existing issues, open and closed, and I'm convinced that mine is new.
[x] When the request is meant for an existing plugin, I've added its name to the title.

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

I would like to have a gRPC streaming handler, with the TLS managed by Caddy. For this the upstream connection must use h2c (gRPC streams require http/2, but I don't want to have a certificate, hence "unencrypted http/2" a.k.a. h2c).

According to the Caddy docs, the resulting config should look like:

reverse_proxy {
    to h2c://127.0.0.201:7777
}

Describe the solution you'd like I would like to be able to select h2c:// as upstream protocol (currently only http:// and https:// are available).

Describe alternatives you've considered Using a Layer4 config should work, but I am not sure how to enable the TLS termination.

Additional context

The template should be adapted around the following lines:

https://github.com/opnsense/plugins/blob/8cd1be1adbf33c4d04d4b77406c0b84f1de0a42e/www/caddy/src/opnsense/service/templates/OPNsense/Caddy/Caddyfile#L447

The model should be expanded as well:

https://github.com/opnsense/plugins/blob/8cd1be1adbf33c4d04d4b77406c0b84f1de0a42e/www/caddy/src/opnsense/mvc/app/models/OPNsense/Caddy/Caddy.xml#L358

cc @Monviech who originally created the Caddy plugin, I believe (thanks for the great work BTW!)

opnsense / plugins

Caddy: support h2c protocol for reverse_proxy #4368