Opaque PyObject ABI support

[ngoldbaum]

Fixes #5786.

• Adds support for the new PEP 803 abi3t ABI.
  • Adds new abi3t and abi3t-py315 features to enable abi3t builds. These features work analogously to the abi3 features.
• Adds a new target_abi field to the InterpreterConfig struct. This allows keeping track of a separate host and target Python version and fixing some longstanding issues with separation of concerns there.
• Adds a PythonAbi and PythonAbiBuilder struct which wraps an implementation, version, and a new PythonAbiKind enum. The PythonAbiKind enum can itself be one of two enums: StableAbi or VersionSpecific. The former can be Abi3 or Abi3t and the latter can be FreeThreaded or GilEnabled. It's a lot of new types but it also allows us to track details about the target ABI in a much more fine-grained manner.

[davidhewitt]

Sorry for the slow review cycle, was busy this week at the Rust All Hands.

I think this is looking great and almost ready to merge. Comments are primarily cleanups, with the most involved suggestion being to gate append_to_inittab.

[ngoldbaum]

I was looking at trying to get the maturin integration tests passing on 3.15t today and noticed that cargo build --features=abi3-py39,abi3t-py315 dies inside the pyo3-ffi build script because the detected target ABI is abi3t-py39, which is definitely wrong.

[ngoldbaum]

I'm trying to stabilize the Maturin tests but that requires pushing here, sorry for the noise. The latest discovery was that zig cross-compilation was broken which indicates I probably need to add more unit tests for generating an InterpreterConfig for a free-threaded target.

[ngoldbaum]

It looks like Maturin's CI is now stable with all test crates updated to use this PR and CI running on both 3.15 interpreters.