Install rtcompat automatically

[douglas-raillard-arm]

Description

If polars install process could detect when rtcompat is needed automatically, that would make it a lot easier to deploy to legacy hardware.

One way of achieving that would be to convert polars' pyproject.toml to setup.py. Then the same code that currently detects CPU features could be called from setup.py to add polars-runtime-compat to the required dependencies. The code could also check for POLARS_PREFER_PKG and POLARS_FORCE_PKG to honor explicit end users' requests. This would also allow not installing the non-compat runtime for when it is not needed.