feat(source): protobuf field presence

[xiangjinwu]

Just opening an issue to acknowledge this has not been supported yet.

Without field presence (default), missing fields from a message would be filled with their default value (e.g. 0 or '') rather than null. This used to be the common practice of protobuf.

However in some cases, it may be desired to use null for unset fields and only ingest 0/'' when the producer explicitly set them to this value.

This is related to optional and oneof in protobuf specification.

https://protobuf.dev/programming-guides/field_presence/

[github-actions]

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