Support for remote audio volume control is optional for HSP and HFP profiles, and there is no feedback from peer for +VGM and +VGS.
In practice remote volume control is implemented for speaker and changing volume using +VGS reply works, while doing the same with +VGM for mic does not.
Assume software volume control for peer mic until peer sends AT+VGM= command which should mean peer supports remote volume control for mic as well.
This change is based on Rodrigo Araujo's work here https://lists.freedesktop.org/archives/pulseaudio-discuss/2017-September/028820.html
Note the new can_attenuate_source_volume API change for transport also works for A2DP profiles. For A2DP new callbacks are not defined which indicates A2DP transport endpoints cannot do volume attenuation which turns on software volume control.