Fix protomodule.NewMessage handling of protobuf field presence

[ryanpbrewster]

I'm running into an issue where going back and forth between protos and JSON is losing information.

Within the protobuf ecosystem, it's common to use google.protobuf.Value (https://protobuf.dev/reference/protobuf/google.protobuf/#value) to represent JSON types.

In such scenarios, what should

pb = proto.package("google.protobuf")
proto.decode_json(pb.Struct, '{"foo":""}')

return? Today, it returns an empty protobuf, with no information about the key "foo".

The underlying issue here is tracking field presence. I've left a few comments and links to the reference (here).

There was actually a TODO left in the code about this. In an attempt to minimize the potential impact, I've left the existing isFieldSet check and adding a specific carve-out for fields with explicit presence. If desired, I can remove that too — as noted in the previous TODO, Range should handle this automatically, and my testing bears this out.

    // Range iterates over every populated field in an undefined order,
    // calling f for each field descriptor and value encountered.
    // Range returns immediately if f returns false.
    // While iterating, mutating operations may only be performed
    // on the current field descriptor.

[CLAassistant]

All committers have signed the CLA.