[Python] - Improve Model Deserialization Perf

[Kashif Khan (kashifkhan)]

This PR focusses on improving the performance of the deserialization path of python generated files. I wanted to focus on a couple areas where speed ups could be done and see some results

• Do less work :) - today, when a model is built from a parsed payload, every value was sent to _serialize. Things like int, float, list[str] don't need to be serialized. Store them as is and work on things that need it
• Look things up once, not every time — we compute two things one time per model class: a rest_name2field map, and the small list of fields that have client defaults. That way building each object skips re-scanning all the fields
• Annotation Cache - figuring out how to deserialize a field (e.g.  List[Pet],  Optional[datetime]) means walking typing internals, and the answer never changes for a given type, so it's cached.

On a ~5.6 MB DocumentIntelligence-shaped (why I started this) response on Python 3.10:

• Building the model tree: ~2.7× faster (~880 ms dropped to ~330 ms)
• Build + read every field: roughly halved (~2.0 s dropped to ~1.15 s)

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npm i https://pkg.pr.new/@typespec/http-client-python@11704

commit: d85b447

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[Kashif Khan (kashifkhan)]

new branch here #11705