Mechanism to provide a transformation features to libwebrtc

[k-wasniowski]

There are couple of ways in which transformation features could be negotiated:

Option 1.
Set transform method with additional argument passing requested features:

SetFrameTransformer(transformer, set<features>

For now only sframe feature would be supported.
Passing in a new set of features would trigger ONN event.

Option 2.
SetFrameTransform always triggers ONN.
Extend transformer interface implementation listening for a event triggered by createOffer/answer.
Event would enable modifying MediaDescriptionOptions which will be used later to generate specific m-line.