Add initial out-of-Matrix login system

example-config.yaml

@@ -14,6 +14,18 @@ appservice:
     hostname: localhost
     port: 8080
 
+    # Public part of web server for out-of-Matrix interaction with the bridge.
+    # Used for things like login if the user wants to make sure the 2FA password isn't stored in
+    # the HS database.
+    public:
+        # Whether or not the public-facing endpoints should be enabled.
+        enabled: true
+        # The prefix to use in the public-facing endpoints.
+        prefix: /public
+        # The base URL where the public-facing endpoints are available. The prefix is not added
+        # implicitly.
+        external: https://example.com/public
+
     # Whether or not to enable debug messages in the console.
     debug: false