Add C# plugin "file_suffix" option defaulting to "Grpc.cs" (#26162)

* add C# plugin "file_extension" option defaulting to "Grpc.cs"

* rename to "file_suffix" to avoid confusion

* address review feedback

src/compiler/csharp_generator_helpers.h

@@ -25,9 +25,10 @@
 namespace grpc_csharp_generator {
 
 inline bool ServicesFilename(const grpc::protobuf::FileDescriptor* file,
-                             std::string* file_name_or_error) {
-  *file_name_or_error =
-      grpc_generator::FileNameInUpperCamel(file, false) + "Grpc.cs";
+                             const std::string& file_suffix,
+                             std::string& out_file_name_or_error) {
+  out_file_name_or_error =
+      grpc_generator::FileNameInUpperCamel(file, false) + file_suffix;
   return true;
 }
 

src/compiler/csharp_plugin.cc

@@ -43,6 +43,9 @@ class CSharpGrpcGenerator : public grpc::protobuf::compiler::CodeGenerator {
     bool generate_client = true;
     bool generate_server = true;
     bool internal_access = false;
+    // the suffix that will get appended to the name generated from the name
+    // of the original .proto file
+    std::string file_suffix = "Grpc.cs";
     for (size_t i = 0; i < options.size(); i++) {
       if (options[i].first == "no_client") {
         generate_client = false;
@@ -50,6 +53,9 @@ class CSharpGrpcGenerator : public grpc::protobuf::compiler::CodeGenerator {
         generate_server = false;
       } else if (options[i].first == "internal_access") {
         internal_access = true;
+      }
+      if (options[i].first == "file_suffix") {
+        file_suffix = options[i].second;
       } else {
         *error = "Unknown generator option: " + options[i].first;
         return false;
@@ -64,7 +70,8 @@ class CSharpGrpcGenerator : public grpc::protobuf::compiler::CodeGenerator {
 
     // Get output file name.
     std::string file_name;
-    if (!grpc_csharp_generator::ServicesFilename(file, &file_name)) {
+    if (!grpc_csharp_generator::ServicesFilename(file, file_suffix,
+                                                 file_name)) {
       return false;
     }
     std::unique_ptr<grpc::protobuf::io::ZeroCopyOutputStream> output(