Per requirements `--config-settings`

[sbidoul]

What's the problem this feature will solve?

Currently we can specify --install-options and --global-options per requirement in requirement files.

These options are on the way to deprecation as they trigger the legacy setup.py install code path.

Towards https://github.com/pypa/pip/issues/8102.

Describe the solution you'd like

Allow --config-settings on requirement lines so they can be passed to the build backend.

Evaluate automatically activating PEP 517 build for requirements with such options.

Alternative Solutions

Do not allow configuring build settings per requirements.

Additional context

https://github.com/pypa/pip/pull/9422#issuecomment-1116047542

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