utcDateTime defaults to Protobuf.WellKnown.Timestamp

[tjb1982]

This makes TypeSpec.utcDateTime default to Protobuf.WellKnown.Timestamp by mapping utcDateTime to ref("google.prototype.Timestamp") making sure to import "google/protobuf/timestamp.proto".

This required the creation of a new type ProtoTypeInfo wrapping a ProtoType with source info to communicate to addType that this particular scalar is actually a reference that needs to be imported.

As reported in #1882

[tjb1982]

@microsoft-github-policy-service agree company="nclk"

[github-actions]

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• ❌@typespec/protobuf

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