[gRPC] Add new options to control the gRPC code generation. (#8298)

The new options are:

-  `--grpc-filename-suffix` controls the suffix of the generated files;
-  `--grpc-use-system-headers` controls the type of C++ includes generated;
-  `--grpc-search-path` controls the directory that contains gRPC runtime;
-  `--grpc-additional-header` allows to provide additional dependencies for the generated code.

grpc/src/compiler/cpp_generator.h

@@ -35,6 +35,8 @@ struct Parameters {
   bool generate_mock_code;
   // By default, use "_generated.h"
   std::string message_header_extension;
+  // Default: ".grpc.fb.h"
+  std::string service_header_extension;
 };
 
 // Return the prologue of the generated header file.