s3: add grpc server to accept configuration changes

weed/pb/s3.proto

@@ -0,0 +1,25 @@
+syntax = "proto3";
+
+package messaging_pb;
+
+option go_package = "github.com/chrislusf/seaweedfs/weed/pb/s3_pb";
+option java_package = "seaweedfs.client";
+option java_outer_classname = "S3Proto";
+
+//////////////////////////////////////////////////
+
+service SeaweedS3 {
+
+    rpc Configure (ConfigureRequest) returns (ConfigureResponse) {
+    }
+
+}
+
+//////////////////////////////////////////////////
+
+message ConfigureRequest {
+    bytes configuration_file_content = 1;
+}
+
+message ConfigureResponse {
+}