volume: add grpc file read operation

This is added more for performance benchmarking.

weed/pb/volume_server.proto

@@ -8,6 +8,10 @@ service VolumeServer {
     //Experts only: takes multiple fid parameters. This function does not propagate deletes to replicas.
     rpc BatchDelete (BatchDeleteRequest) returns (BatchDeleteResponse) {
     }
+
+    rpc FileGet (FileGetRequest) returns (stream FileGetResponse) {
+    }
+
     rpc VacuumVolumeCheck (VacuumVolumeCheckRequest) returns (VacuumVolumeCheckResponse) {
     }
     rpc VacuumVolumeCompact (VacuumVolumeCompactRequest) returns (VacuumVolumeCompactResponse) {
@@ -100,6 +104,22 @@ message DeleteResult {
     uint32 version = 5;
 }
 
+message FileGetRequest {
+    string file_id = 1;
+    bool accept_gzip = 2;
+}
+message FileGetResponse {
+    bytes data = 1;
+    uint32 content_length = 2;
+    string content_type = 3;
+    uint64 last_modified = 4;
+    string filename = 5;
+    string etag = 6;
+    bool is_gzipped = 7;
+    map<string, string> headers = 8;
+    int32 errorCode = 9;
+}
+
 message Empty {
 }