Authenticators: Add BasicAuth operation and handle it in transaction

This adds a new way for authenticators to trigger interactive authentication
in the client for simple user/password dialogs. These are not recommended
to use as a webflow is often better. However, for OCI remotes that
use http basic auth this is useful and allows the CLI experience to work
with that.
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f