security/rubygem-auth-sanitizer: Add rubygem-auth-sanitizer 0.1.2

auth-sanitizer provides small, dependency-light helpers for keeping OAuth and
authentication secrets out of object inspection and log output.

The gem is intentionally narrow in scope. It does not change HTTP requests,
token objects, persistence, or application configuration for you. Instead, it
gives host gems and applications two reusable redaction surfaces:

• Auth::Sanitizer::FilteredAttributes redacts selected instance variables from #inspect.
• Auth::Sanitizer::SanitizedLogger wraps an existing logger and redacts sensitive values from string log messages.

(cherry picked from commit 4bbca23bd638147c61ce5fa1963286e8aad46644)