Reduce the binary size

[gsdevme]

Ideally the binary size would be the smallest possible, because Caddy is being used we are pulling lots of dependencies we don't strictly need as the fastcgi package has lots of dependencies.

Possible options

1. See if Caddy will extract it into its own package, might be sensible as its an implementation against a 3rd party specification anyway Move Fastcgi client implementation to its own package caddyserver/caddy#4378
2. Copy the contents of the file (less ideal... also just wrong)
3. Git Submodule? 🤷 not ideal.
4. Something go mod can do I don't know about?