Add a new refresh message to request a keyframe from the pinos server.
pinospay: pass the refresh-request message upstream
pinossink: turn refresh-request messages into events
pinossrc: turn a keyframe event into a refresh-request message
Rework stream connect/disconnect from start/stop
Start the stream in setcaps when the format is known.
When the stream is already started in setcaps, send a format_change
message instead.