libobs:  Add 'initialize' callback to services

The 'initialize' callback is used before the encoders/output start up so
it can adjust encoder settings to required values if needed.

Also added the function 'obs_encoder_active' that returns true or false
depending on whether that encoder is active or not.
rtmp-common: Apply service encoder settings

Common services that require specific encoder settings (specified in the
'recommended' section of the json file) will now have those encoder
settings applied before the encoders start up.

This fixes the issue with services like twitch that weren't getting
their required 2-second keyframe maximum interval set.