Migrate StableHLO Python extension to nanobind.

I'm working towards moving the MLIR Python core code to use nanobind instead of pybind11:
* llvm/llvm-project#117922, which was merged recently, allows downstream Python dialect extensions to be defined using either pybind11 or nanobind.
* llvm/llvm-project#118583 is a PR in review that ports the Python core code to use nanobind instead of pybind11.

This PR migrates StableHLO and related dialects to use nanobind rather than pybind11, with the goal of migrating JAX away from pybind11.

PiperOrigin-RevId: 705197067

Author: hawkinsp