Python 3.15 final will allow including platform tags in stable abi extension filenames

[ngoldbaum]

See python/cpython#152461

Currently PyO3 doesn't do that. The tagless extensions are also supported but since e.g. Debian will likely want it we should probably support it as well.

[ngoldbaum]

There aren't many needed changes here. In principle we could do nothing, but I think, as a new feature, we should probably add an InterpreterConfig::soabi_filename() accessor that gets written to and read from build configuration files. That way downstream consumers don't have to reimplement all that logic.

Maturin doesn't depend on pyo3-build-config, so it'll need its own independent implementation.

Also note that the backport PR hasn't landed yet and this isn't actually available in a released beta or RC of 3.15 at time of writing.