src/daemon: move the optional drop-ins to conf.avail

These drop-ins are not meant to be enabled by default, so let's move
them to fontconfig style *.conf.avail/ subfolders from which they can
be copied or symlinked to a location that will get merged into the
corresponding configuration file.

Signed-off-by: Niklāvs Koļesņikovs <89q1r14hd@relay.firefox.com>

src/daemon/client.conf.avail/20-upmix.conf.in

@@ -0,0 +1,8 @@
+# Enables upmixing
+stream.properties = {
+    channelmix.upmix      = true
+    channelmix.upmix-method = psd  # none, simple
+    channelmix.lfe-cutoff = 150
+    channelmix.fc-cutoff  = 12000
+    channelmix.rear-delay = 12.0
+}

src/daemon/client.conf.avail/meson.build

@@ -0,0 +1,11 @@
+conf_files = [
+ '20-upmix.conf',
+]
+
+foreach c : conf_files
+  configure_file(input : '@0@.in'.format(c),
+                 output : c,
+                 configuration : conf_config,
+                 install_dir : pipewire_confdatadir / 'client.conf.avail')
+endforeach
+