datalad_next.patches.create_sibling_ghlike

Improved credential handling for create_sibling_<github-like>()

This patch makes the storage of a newly entered credential conditional on a successful authorization, in the spirit of datalad/datalad#3126.

Moreover, stored credentials now contain a realm property that identified the API endpoint. This makes it possible to identify candidates of suitable credentials without having to specific their name, similar to a request context url used by the old providers setup.

This automatic realm-based credential lookup is now also implemented. When no credential name is specified, the most recently used credential matching the API realm will be used automatically. If determined like this, it will be tested for successful authorization, and will then be stored again with an updated last-used timestamp.