feat: Add trace context propagation API for Node.js SDK (parity with Python)

[golergka]

Problem

When building production agents with the Node.js SDK, I need to correlate my application's OpenTelemetry spans with LiveKit's internal spans (agent_session, agent_turn, user_turn_completed).

Currently, the Python SDK exposes set_tracer_provider() for trace context configuration, but the Node.js SDK has no equivalent API. This means:

1. LiveKit's spans and my application's spans have separate trace IDs
2. I cannot visualize the full call flow in a single trace tree
3. Debugging requires manually correlating spans by room_sid across two separate traces

Current Behavior

• @livekit/rtc-node registers an OpenTelemetry TracerProvider at import time
• My observability code can add span processors to this provider (so spans export to the same backend)
• But there's no way to share trace context - each creates its own root span

Desired Behavior

An API similar to Python's set_tracer_provider() that allows:

1. Setting a custom TracerProvider, OR
2. Propagating a parent span context so LiveKit's spans become children of my trace

Use Case

I have an agent with an 8-phase initialization pipeline (init:entry, init:connect_room, init:prepare_session, etc.). I want all of these phases PLUS LiveKit's internal spans to appear in a single trace tree for end-to-end observability.

Environment

• @livekit/agents: v1.0.x
• Node.js: v22.x
• OpenTelemetry SDK: v2.x