refactor(core): context stage 1

[Archento]

This is the first iteration of Context refactoring where all agent related properties within the context are clustered to one ctx.agent property. This is possible with the new AgentRepresentation class and enables lean passthrough of necessary properties without exposing unnecessary interfaces.

[jrriehl]

Looks good to me. Since there are a few breaking changes here, I think we just need to compile a list of them somewhere that we can include in release notes, docs, etc.