[Feature request] Allow to use metacontroller as a library

[grzesuav]

Use case

Sometimes might be more convenient to extend metacontroller as a library.

Todo

• define metacontroller API and internal packages

[mkmik]

Is this about having a mode where metacontroller and the custom controller can run in the same process and metacontroller, no need for metacontroller custom resources and having metacontroller not have to call the webook over the network?

Or is this just to run metacontroller + custom controller in the same process so that metacontroller can invoke the webhook over localhost?