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.

data/org.freedesktop.Flatpak.Authenticator.xml

@@ -185,6 +185,28 @@
     -->
     <signal name="WebflowDone">
     </signal>
+    <!--
+        BasicAuth:
+
+        Emitted by the authenticator when it needs to do a simple user + password authentication.
+        This is only useful for very simple authentication interaction, but this is still used (for
+        instance for http basic access authentication), and for those cases this allows a nicely
+        integrated UI and CLI experience.
+    -->
+    <signal name="BasicAuth">
+      <arg type="s" name="realm"/>
+    </signal>
+    <!--
+        BasicAuthReply:
+        @user: The user
+        @password: The password
+
+        Call to finish the request started with the BasicAuth signal.
+    -->
+    <method name="BasicAuthReply">
+      <arg type="s" name="user"/>
+      <arg type="s" name="password"/>
+    </method>
     <!--
         Response:
         @response: Numeric response