Support for different payload types

[mat-hek]

Currently we use only one payload type - erlang binary. We should support also a native C payload type, mutable and passed by pointer. This is necessary for processing large amount of data (such as high quality video).
We should consider:

• Way of passing pointer to different NIFs
• Monitoring lifecycle of payload
• Adding payload type to caps
• Size of each payload type should be accessible from elixir with possibly low overhead, because its used frequently to handle demand-related stuff.
• Payload should be automatically converted by core if type is not supported by element.

[mspanc]

This is a strict requirement for adding video so according to the planning meeting on 25.06.2018 it should be done ASAP.

[mspanc]

Caps should contain information about the payload type that may arrive to the element.