ast_format: Adds an identifier for interleaved audio formats to the ast_format

Adds an identifier (with a getter and setter) to detect channels with
interleaved audio.
This is needed by the binaural bridge_softmix patch (ASTERISK-26292) and
was already discussed here:
http://lists.digium.com/pipermail/asterisk-dev/2016-October/075900.html
The identifier can be set during fmtp parsing (to be seen in the
res_format_attr_opus.c change).

ASTERISK-26292

Change-Id: I359801cc5f98c35671c48dabc81a7f4ee1183d63