[CMake] Abilty to pass options to protoc executable from cmake

[igor-sirotin]

Added PROTOC_OPTIONS argument to protobuf_generate cmake-function.
This gives ability to pass any required options to protoc executable, e.g. --experimental_allow_proto3_optional, or --proto-path.

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[igor-sirotin]

@googlebot I signed it!

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[igor-sirotin]

Is there anything else I need to do?

[acozzette]

Thanks, @igor-sirotin.