Ensure that a PushProvider is available on a device before using it.

It help to fallback to UnifiedPush (if available) if the PlayServices are not installed on the device.

libraries/pushproviders/api/src/main/kotlin/io/element/android/libraries/pushproviders/api/PushProvider.kt

@@ -32,6 +32,11 @@ interface PushProvider {
      */
     val name: String
 
+    /**
+     * Return true if the push provider is available on this device.
+     */
+    fun isAvailable(): Boolean
+
     fun getDistributors(): List<Distributor>
 
     /**