[Compiler] Make the code generator pointer mutable in PluginMain (#34170)

This will allow us to pass some extra initialization information from
the protobuf plugin code before the build actually starts

src/compiler/config.h

@@ -40,8 +40,7 @@ namespace protobuf {
 namespace compiler {
 typedef GRPC_CUSTOM_CODEGENERATOR CodeGenerator;
 typedef GRPC_CUSTOM_GENERATORCONTEXT GeneratorContext;
-static inline int PluginMain(int argc, char* argv[],
-                             const CodeGenerator* generator) {
+static inline int PluginMain(int argc, char* argv[], CodeGenerator* generator) {
   return GRPC_CUSTOM_PLUGINMAIN(argc, argv, generator);
 }
 static inline void ParseGeneratorParameter(