feat(git config): Set default user.name and user.email in git config

[chouetz]

Hello 👋

This PR is a (yet another) tentative to have a solution to issue #13 in the action/checkout Github Action.
From the last comments on this issue, it seems everybody agreed on such a setting on this action. I've found a PR #584 on this topic opened 3y ago and which didn't applied the last requested suggestions.
So I decided to do an additional proposal with:

• addition of single boolean input to set a default configuration, as proposed here
• update of the configuration as done on PR Set default user.name and user.email #584, using the default values of the suggestion, to answer the question raised on the initial issue.

I'll try to apply any suggestion to this PR so that we can close #13.

Thanks in advance