package/alsa-plugins: new package

package/alsa-plugins: new package

During testing of bluez-alsa in particular, there is no compilation
dependency - but, some configurations will request the load of a
particular plugin:

    ALSA lib dlmisc.c:337:(snd_dlobj_cache_get0) \
      Cannot open shared library libasound_module_rate_samplerate_best.so \
      (/usr/lib/alsa-lib/libasound_module_rate_samplerate_best.so: \
       cannot open shared object file: No such file or directory)

Adding in the alsa-plugins package allows to build build the
appropriate plugins.

Signed-off-by: Charles Hardin <ckhardin@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>