alsa: Add a varlink interface definition for external volume control

The interface itself isn't really ALSA-specific, but we can move it out
of we find other uses for it.

spa/plugins/alsa/acp/org.pipewire.ExternalVolume.varlink

@@ -0,0 +1,89 @@
+# Allows an external service to provide volume control for a PipeWire device.
+interface org.pipewire.ExternalVolume
+
+# Describes what kind of volume control operations are supported for a given
+# device.
+type Capabilities (
+  # Whether the current volume value can be read
+  readVolume: bool,
+
+  # Whether volume values are reported per-channel
+  readBalance: bool,
+
+  # The range of valid volume values and the granularity of steps
+  volumeRange: (
+    min: float,
+    max: float,
+    step: float
+  ),
+
+  # Whether the volume can be set as an absolute value
+  writeVolumeAbsolute: bool,
+
+  # Whether volume adjustments can be made relative to the current volume
+  writeVolumeRelative: bool,
+
+  # The size of relative volume adjustments, if known
+  writeVolumeRelativeStep: (min: float, max: float),
+
+  # Whether per-channel volumes can be written
+  writeBalance: bool,
+
+  # Whether the current mute state can be read
+  readMute: bool,
+
+  # Whether the current mute state can be set
+  writeMuteValue: bool,
+
+  # Whether the current mute state can be toggled
+  writeMuteToggle: bool,
+
+  # The known set of routes for the device
+  routes: []string
+)
+
+# Query volume control capabilities for the given device.
+method GetCapabilities(device: string) -> (capabilities: Capabilities)
+
+# Query the volume for the given device route. If the volume can be read, the
+# returned value will be an array of floats (if per-channel volumes are not
+# supported, the array will have one float value).
+method ReadVolume(device: string, route: string) -> (volume: []float)
+
+# Query the mute state for the given device route.
+method ReadMute(device: string, route: string) -> (mute: bool)
+
+# Monitor changes to volume or mute state. Volume changes will be signalled by
+# a non-empty volume array (with a single value if per-channel volumes are not
+# supported). Mute state changes will be signalled by a non-null mute value.
+method Monitor(device: string) -> (
+  route: string,
+  volume: []float,
+  mute: ?bool
+)
+
+# Set the volume of the given device route. If supported, the provided value
+# can be an array of per-channel float values. If per-channel volumes are not
+# supported, the array should consist of a single value.
+method WriteVolumeAbsolute(
+  device: string,
+  route: string,
+  volume: []float
+) -> ()
+
+# Increase or decrease the volume of the device route by the given amount.
+method WriteVolumeRelative(
+  device: string,
+  route: string,
+  step: float
+) -> ()
+
+# Set the mute state for the given device route.
+method WriteMuteValue(
+  device: string,
+  route: string,
+  mute: bool
+) -> ()
+
+# Toggle the mute state for the given device route.
+method WriteMuteToggle(device: string, route: string) -> ()