Identify invalid/unknown options or environment variables via "pip config debug"

[deveshks]

Now that #6741 has been closed, a follow up for that has been requested in #8096 (review)

To reiterate, we should be able identify options specified in pip config files which are invalid, optionally trying to identify the closest option available to the provided option, and suggesting it to the user.

For e.g. PIP_TOTO=titi pip config debug should state that TOTO is an invalid option, but for something like global.timout: 10, we can both state that timout is an invalid option, but timeout is a suggestion which can be suggested.

We should ideally discuss first on whether this is a good enough extension of debug subcommand of pip config to be implemented, and come up with a rough idea to implement it, and iterate on it over a PR.