Enable x-accel-redirect

[withinboredom]

closes #365

This adds x-accel-redirect and friends by passing a "filter" to Frankenphp from Caddy. This "filter" allows us to halt output if we have a rewriting matcher that should take over the request. For backward compatibility, context strings are kept as http.reverse_proxy.*.

This is a refactoring of the reverse proxy functionality built into Caddy but adapted for FrankenPHP.

        php_server {
            @accel header X-Accel-Redirect *
            handle_response @accel {
              root * /
              rewrite {http.reverse_proxy.header.X-Accel-Redirect}
              file_server
            }
        }

Todo:

• [x] worker support
• [x] tests
• [x] clean up debug log lines
• [x] return docker files to a pristine condition
• [x] refactoring
• [x] documentation (for caddyfile)
• [x] documentation (for frankenphp -- hooks)

Thoughts:

• Anything other than handle_response is probably out of scope for this PR.

[dunglas]

Hey @withinboredom, do you mind if I push in your branch to finish this feature?

[withinboredom]

Yeah, go for it. I just haven't had much time to get to this.

[dunglas]

Here is my alternative PR (directly in Caddy): https://github.com/caddyserver/caddy/pull/6232

[dunglas]

Closing in favor of https://github.com/caddyserver/caddy/pull/6232 that is likely to be merged. Thanks for paving the way @withinboredom!!

[withinboredom]

🥳