alsa: Allow sample spec override in mappings

This allows mappings to override some or all of the sample_spec used to
open the ALSA device. The intention, to start with, is to use this for
devices in UCM that need to be opened at a specific rate (like modem
devices). This can be extended to allow overrides in profile-sets as
well.

src/modules/alsa/alsa-mixer.h

@@ -251,6 +251,7 @@ struct pa_alsa_mapping {
     /* These are copied over to the resultant sink/source */
     pa_proplist *proplist;
 
+    pa_sample_spec sample_spec;
     pa_channel_map channel_map;
 
     char **device_strings;