Support codec G726 for Airoha
This is a replacement of MR: !24 (closed) As the old MR has a compilation issue and we don't have access to the original branch.
asterisk-chan-voicemngr uses old G726-32 RTP playload type '2' which
is different from the type '80' used by Airoha DSP.
RFC3551 has re-defined RTP payload type of G726-32 to "dynamic", and
payload type '2' as "reserved".
1) Refactory the codec related functions as table-driven.
2) Retrieve the RTP payload type from Airoha endpoint (provided by
libvoice-airoha) via ubus call endpt codecs (attribute "rtp_payload").
Received RTP payload type is applied, otherwise old one.