Add awaitable_attrs to rx.Model

[masenf]

This works like sqlalchemy's AsyncAttrs mixin, providing an async-context for accessing fields and relationships when using async sessions.

[codspeed-hq]

CodSpeed Performance Report

Merging #4778 will not alter performance

_{Comparing masenf/rx-model-awaitable-attrs (5411cc0) with main (70920a6)}

Summary

✅ 8 untouched benchmarks