Beginning of extensibility via modules

This is an important refactoring to make it easier to add new functionality to both kas and agentk. Also, splitting separate concerns into separate "modules" makes it much easier to test it all.

I've only refactored the core of agentk here, didn't add more tests or refactor more things to keep it smaller. Will do in follow up MRs.

See !179 (merged) for a bigger picture.