Support go plugins

[qwerty287]

Support go plugins to allow extending woodpecker without changes to the main code.

Example features:

• forges
• agent backends
• secret providers
• config services
• ...

https://pkg.go.dev/plugin

[qwerty287]

I just started working on this and already got a working prototype that allows to import external forges. I'll see when I can finish it and open a PR.