feat: support conditional cargo features based on Python version

[messense]

Extend [tool.maturin] features to accept both plain feature strings and conditional objects with feature and python-version keys:

features = [
  "always-on-feature",
  { feature = "pyo3/abi3-py311", python-version = ">=3.11" },
  { feature = "pyo3/abi3-py38", python-version = "<3.11" },
]

Conditional features use PEP 440 version specifiers and are resolved at build time against the target Python interpreter version.

Closes #2472

[Copilot]

Pull request overview

This PR extends tool.maturin.features to support conditional Cargo features that are enabled based on the target Python interpreter version using PEP 440 version specifiers.

Changes:

• Added a FeatureSpec type to parse either plain feature strings or conditional { feature, python-version } objects.
• Plumbed conditional feature extraction into BuildContext and resolve/enable them during Cargo build command construction.
• Updated JSON schema and documentation, and added tests for parsing and resolution.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/pyproject_toml.rs	Introduces FeatureSpec, updates ToolMaturin.features, and adds tests for conditional feature parsing/resolution.
src/compile.rs	Resolves conditional features from BuildContext using the target interpreter and appends them to Cargo features.
src/build_options.rs	Extracts conditional feature rules from pyproject.toml and ensures only plain features become base Cargo features.
src/build_context.rs	Stores conditional feature rules on BuildContext.
maturin.schema.json	Extends schema to allow mixed string/object feature entries via FeatureSpec.
guide/src/config.md	Documents conditional features under [tool.maturin].