Add TryFrom implementations used in Spector tests

[antkmsft]

The Spector test using the syntax: https://github.com/Azure/typespec-rust/pull/502

[LarryOsterman]

I'm going to ask an ugly question: Do we really need to support a Decimal type for any scenarios other than Spector?

Could this be implemented using a decimal feature to make the dependency an optional dependency so we don't add this package (and its half dozen or so unique dependencies) to the SDK? That way service clients which need the Decimal type can add the typespec_client_core with the decimal feature but other clients won't have the overhead.