ALSA Hook PCM for HFP-AG support

[borine]

This is a tentative proposal, more a proof-of-concept or discussion piece than a finished work.

Issue #266 proposes adding basic HFP call transfer and termination support to the BlueALSA server when used with HFP-AG profile. This PR addresses the same requirement by adding optional extra functionality to the ALSA client.

I've deliberately kept the new function completely separated from the existing client by implementing it as a Hook PCM with a distinct config file: this is to simplify discussion, experimentation and initial testing. If the new functionality is deemed desirable then the implememtation could possibly be made more efficient by integrating it with the existing ALSA client.

There's no documentation yet, but the idea is that the user is given a new pre-defined PCM called "hfp" that adds call session handling and is used in much the same way as the bluealsa PCM:

aplay -D hfp sounds.wav
arecord -D hfp:DEV=XX:XX:XX:XX:XX:XX,CODEC=msbc -r 16000 -c 1 -f s16_le recording.wav

[borine]

I've just force-pushed a re-factored version which will be easier to add into the bluealsa pcm plugin, should that be desired, and hopefully is also more robust

[codecov[bot]]

Codecov Report

Base: 69.13% // Head: 62.40% // Decreases project coverage by -6.72% :warning:

Coverage data is based on head (4c1a12a) compared to base (d35297c). Patch has no changes to coverable lines.

:exclamation: Current head 4c1a12a differs from pull request most recent head b93770a. Consider uploading reports for the commit b93770a to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #525      +/-   ##
==========================================
- Coverage   69.13%   62.40%   -6.73%     
==========================================
  Files          44       32      -12     
  Lines        7564     6544    -1020     
==========================================
- Hits         5229     4084    -1145     
- Misses       2335     2460     +125     

Impacted Files	Coverage Δ
src/hfp.c	33.33% <0.00%> (-58.34%)	:arrow_down:
src/bluealsa-skeleton.c	67.56% <0.00%> (-32.44%)	:arrow_down:
src/bluealsa-dbus.c	37.61% <0.00%> (-30.85%)	:arrow_down:
src/shared/dbus-client.c	54.30% <0.00%> (-17.41%)	:arrow_down:
src/dbus.c	32.69% <0.00%> (-15.39%)	:arrow_down:
src/asound/bluealsa-pcm.c	71.31% <0.00%> (-4.16%)	:arrow_down:
src/asound/bluealsa-ctl.c	71.83% <0.00%> (-4.08%)	:arrow_down:
src/a2dp.c	39.93% <0.00%> (-3.87%)	:arrow_down:
src/ba-transport.c	72.59% <0.00%> (-2.03%)	:arrow_down:
src/io.c	63.29% <0.00%> (-0.67%)	:arrow_down:
... and 24 more

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[borine]

I now feel this product would sit better outside of the bluez-alsa project, and so it is now maintained as a distinct project here: https://github.com/borine/bluealsa-hfp-ag-plugin