Publisher-transport deadlock fixed; NVENC bitrate updates now live

Fixes

• Add a unified EgressClient::start_egress that calls the v2 Egress.StartEgress RPC with a StartEgressRequest, alongside the existing per-type helpers.
• Fix a publisher-transport deadlock during renegotiation. When another negotiation was requested while an offer was awaiting its answer, set_remote_description re-entered create_and_send_offer while holding the transport's non-reentrant inner mutex, permanently wedging publishing.
• Move the concept of "internal" data streams into livekit crate from livekit-data-stream - #1304 (@1egoman)

Fix H.264, H.265, and AV1 NVENC sessions so live bitrate and frame rate updates

reconfigure the hardware without restarting the encoder.