It is common to want to match only a subset of fields in large protobufs, and doing that in rust right now requires manually clearing out the fields in the `actual` message that we don't care about. This is a pain, and a worse experience when compared to C++.
Like in C++, when doing partial matching, only fields set in the expected protobuf message are compared, while extra fields set only in the actual message are ignored.
When using the C++ kernel we leverage `proto2::util::MessageDifferencer`, and for the UPB kernel we use the `kUpb_CompareOption_Partial` option.
PiperOrigin-RevId: 964125682
