Make DirectClient extensible

[maxsap]

At the moment, DirectClient is spinning up its own API endpoints, and creates its own assumptions when it comes to user Ids e.t.c.

This can kill injection of external context to be passed through the API and force the dev's to build custom providers/actions to obtain this same context through conversation.

Example usecase:

• I would like to pass user ids of authenticated users
• with those IDs I can invoke providers to already load my user's profiles and look up information (like wallets, if they have achieved certain milestones e.t.c.)
• then the very first time you chat with the agent you already can start your discussion from a specific point rather than from 0

This can be something generic like setting an enrichment or extenralContext callback in express functions, then the result of this callback can be added to the message as a generic user provided context where you could already get this from providers/actions or whatever other component relies on the message abstraction.