add native methods from alpine-librdkafka.so (#883)

Author: zerda

src/Confluent.Kafka/Impl/LibRdKafka.cs

@@ -436,6 +436,7 @@ public static bool Initialize(string userSpecifiedPath)
                         nativeMethodTypes.Add(typeof(NativeMethods.NativeMethods));
                         nativeMethodTypes.Add(typeof(NativeMethods.NativeMethods_Debian9));
                         nativeMethodTypes.Add(typeof(NativeMethods.NativeMethods_Centos7));
+                        nativeMethodTypes.Add(typeof(NativeMethods.NativeMethods_Alpine));
                     }
                 }
                 else