[gRPC] Add new options to control the gRPC code generation

[anton-bobukh]

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.