pre-connect the publisher PC when an RPC handler is registered

[xianshijing-lk]

Problem:

1. The receiver receives RPC requests through the subscriber data channel (already connected)
2. When sending the ACK, it uses the publisher data channel
3. If this is the receiver's first time sending data, the publisher PC hasn't been negotiated yet
4. ensure_publisher_connected triggers ICE negotiation and polls every 50ms until connected
5. . ICE negotiation with SFU can take 200-500ms, that adds ~400ms delay

This PR fixes the problem

[ladvoc]

Makes sense to me, great find!