Add agent session store protocols

livekit_agent_session_store.proto is the management API (Twirp): create, get,
list and delete a session, plus Dump for a consistent download URL.

livekit_agent_session_db.proto is the data plane: a request/response frame pair
over one WebSocket per session, correlated by client-chosen request_id, with
exec, query, atomic batch, interactive transactions, and credit-paced row
batches so any result size streams with bounded memory at every hop.
SessionValue mirrors SQLite's five storage classes.

The other twirp files change only because protoc-gen-twirp numbers file
descriptors by position in the invocation, so inserting one service shifts the
ones after it.

Author: theomonnom