authenticator: Fix sandboxed authenticators

We rely on broadcast signals for authenticator replies rather than unicast
as these are not filtered by the sandbox (due to them being opt-in by the
receiver).

Actually this already worked fine in the flatpak side as the generated
code already subscribes to the signals, this just switches the internal
authenticators (test and oci) to using the new way to emit signals.
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f