[CMake] Allow custom plugin options e.g. to generate mocks

[tiolan]

This would allow setting options for plugins, e.g. to generate mocks.
Example:

protobuf_generate(
      TARGET
      myTarget
      LANGUAGE
      grpc
      PROTOC_OUT_DIR
      ${CMAKE_BINARY_DIR}
      GENERATE_EXTENSIONS
      .grpc.pb.h
      .grpc.pb.cc
      PLUGIN
      protoc-gen-grpc=$<TARGET_FILE:gRPC::grpc_cpp_plugin>
      PLUGIN_OPTIONS
      generate_mock_code=true
      IMPORT_DIRS
      ${PROTO_IMPORT_DIRS})

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[tiolan]

@googlebot I signed it!

[tiolan]

Resolves #8779

[tiolan]

@acozzette any chance this will be merged?

[acozzette]

Thanks, @tiolan.