Improve ergonomics of ClientRegistry

[hellovai]

Currently users have to do a lot of work to use dynamic clients.

instead we could look into a more type-safe approach such as:

cr = b.ClientRegistry()

cr.ExistingClient.ExtractResume(...)

client = cr.add_llm_client(...)
client.ExtractResume(...)

client = cr.get_llm_client(...)
client.ExtractResume(...)

[aaronvg]

one more note here from discord -- people want to be able to change 1 parameter easily