Handle config_settings as they would be passed by pip

[abravalheri]

This PR is a continuation of the work done to implement PEP 660.

The main objective of this PR is to add support for config_settings to select which mode is going to be used by the build_editable hook.

But since I was already working with it and adding some tests, I ended up fixing other some other items.

Summary of changes

• Improvements in terms of the handling of config_settings:

  • Allow frontends and installers to select the strict editable mode via the config_settings argument passed to the PEP 660 hooks.
    Now users should be able to run:

    pip install -e . --config-settings editable-mode=strict

  • Separate config settings given by --global-option and --build-option. The motivation (and approach) for this change is described in build_meta:__legacy__ config_settings not compatible with pip options #1928.
    When calling setuptools internal commands --global-option is placed before the command name, while --build-option is placed after.
    For the time being the implementation strives to maintain backwards compatibility, however passing all arbitrary arguments via --global-option is now deprecated (a warning is issued).

• Added tests for using config_settings via PEP 660 interface, which also lead to some bug fixing.

• Added "informational warnings" about off-band link tree repository for strict installs.

Closes

Pull Request Checklist

• Changes have tests
• News fragment added in changelog.d/.
  (See documentation for details)