[POC] building Go plugins

[henderkes]

Do not merge this slop, for obvious reasons.

Proof of concept to build shared plugins for caddyserver/caddy#7488

This can be greatly simplified, I just wanted to get something working at first. If the proposed PR gets accepted, I will write this by hand.

General logic should be:

• parse caddy binary for dependency versions
• compile a shared plugin with all shared plugins we want to compile
• parse caddy binary and combined plugin for specific versions, merge them together
• next, when building shared plugin:
• create module importing single plugin, with an empty main method
• go mod tidy
• pin dependencies, direct or indirect that match some of the combined plugin
• compile, rinse and repeat