feat(cli): Add support for completing `--config` argument values with `native-completions`

[nspin]

Add value_hint for the global --config argument to allow completing its value.

The related https://github.com/rust-lang/cargo/pull/16245#issuecomment-3524529383 adds partial support to src/etc/cargo.bashcomp.sh.

What does this PR try to resolve?

Currently, native-completions supports completing the --config flag itself, but not its argument value. This PR adds support for completing its value with a file path (clap::ValueHint::FilePath).

How to test and review this PR?

In a shell supported by native-completions, run:

source <(CARGO_COMPLETE=bash cargo run)

Then, observe that the following completion behaves as expected. That is, the user is prompted with files paths.

cargo --config <tab>

[rustbot]

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