Fixes <https://bugs.gnu.org/31591

>.

* gnu/services/sound.scm (<alsa-configuration>)[alsa-plugins]: New field.
(alsa-config-file): Use 'pcm_type.pulse' and 'ctl_type.pulse' to specify file
paths to the 'pulse' plugin.
* doc/guix.texi (Sound Services): Document this.

Co-authored-by: 宋文武 <iyzsong@member.fsf.org>