Create better interface for set_codecs/get_codecs

[MayamaTakeshi]

The functions: int pjw_get_codecs(char out_codecs); int pjw_set_codecs(const char in_codec_info); should be refactored to receive and return codec data in JSON format.

[MayamaTakeshi]

Something like this:

  codecs: ["pcmu/8000/1", "pcma/8000/1", "speex/16000/1"]

where the order in the array indicates priority.

However, the client might want to specify rtp mapping (codec <=> payload_type). So let's think about this situation too.

[MayamaTakeshi]

Ref: https://rapidjson.org/md_doc_tutorial.html#QueryArray

[MayamaTakeshi]

Supplanted by #43.