Metadata support for Password Rules

For websites where password rules that are already defined using constraint validation attributes can be marked with some metadata so that password managers can understand whether the constraints are standard and obvious for password managers.

This issue is created w.r.t #237

The issue names no files, tests, or entry points. Start by reading the referenced issue #237 and inspect the repository's existing password-rule resources. Done should be a concrete, agreed metadata approach that lets password managers distinguish standard constraints from unclear ones.

Written by the indexing model from the issue text.