Expose a way to access the `PYO3_CONFIG_FILE` values

Some users have specific use cases where they would benefit from being able to access the PYO3_CONFIG_FILE values in some form from the pyo3 crate.

I generally wouldn't advise this, as a lot of these config files (such as library path) are unique to the build environment so wouldn't have any useful meaning in binaries intended to be deployed on other systems.

See original comment from #2000 below.

If anyone has need for this, I'm happy to help review designs and implementation.

It would be nice to have access to the library path from PYO3_CONFIG (or all values) but that is not a problem for me.

E.g. it could be based off the PYO3_CONFIG build mechanism - maybe we could have pyo3_config_var!("lib_path"). Or maybe just pyo3_lib_path!(). I'm open to hearing ideas on what's better.

An alternative could be a module filled with many const values, similar to what built does.

Originally posted by @jonasbb in https://github.com/PyO3/pyo3/issues/2000#issuecomment-979985994

Start by reading the PYO3_CONFIG_FILE/PYO3_CONFIG build mechanism and the discussion in issue #2000; compare the proposed macro and generated-constant approaches. Done means establishing a reviewed design and implementation plan for exposing the configuration values while accounting for build-environment-specific paths.