Package Independent Configuration

[mbway]

I think there is a case for having a config file (e.g. $XDG_CONFIG_HOME/maturin.toml or similar) that a user can use to provide default options that aren't tied to a particular project. This would serve a similar purpose to ~/.cargo/cargo.toml.

There are some options that are specific to a particular project and so only really make sense in Cargo.toml/pyproject.toml such as the features option, but there are others where the user may want a different default to the current one. For example a user might want maturin develop to build in release mode by default.

Also, it would be useful to specify to use uv in maturin develop by default (requested here). This setting would not make sense to specify in Cargo.toml/project.toml since it is a user preference rather than a project setting. This setting would be useful for the maturin-import-hook CI to use uv in the tests.