Add handle_spec_playing

[mat-hek]

This callback would be useful in the following use case:

• The parent spawns children that are supposed to receive a stream
• The parent negotiates with the other party to start sending the stream
• The spawned children may or may not be ready to receive the stream at that point. To be sure, the parent could negotiate the start of the stream in handle_spec_playing

Possibly we would also need handle_spec_setup for the same reason.