BLD: add option to build future-compatible abi3 wheels

[neutrinoceros]

This allows targeting CPython's LimitedAPI (and stable ABI) by setting PYPROJ_LIMITED_API=1 at build time. Resulting wheels are future compatible with yet-unreleased Python versions.
The trade-off is that runtime performance might be impacted, though I suggest measuring it against real load-bearing applications, as I found that in many numpy-based extensions the impact to be negligible or even non-measurable.
I could successfully build the package and run the test suite with this locally.

I suggest a couple possible approaches to leverage this:

• append a cp314-abi3 target to the existing build matrix. Version-specific wheels will still be prefered by installers when available but will default to these otherwise, instead of having to, and often failing to build pyproj from source.
• replace version-specific wheels with a single cp311-abi3 wheel per platform. If there are no visible performance regressions this is definitely the way to go.

[neutrinoceros]

note this comment is going to age like milk because there's a new version of the Limited API coming up in Python 3.15 that's compatible with both GIL-enabled and free-threaded builds. However there are a couple other blockers right now (Cython and setuptools both have work-in-progress patches to support this). Note that I'd be happy to revisit this later this year to add cp315-abi3.abi3t as a target