Add basic end-to-bridge encryption support

Still missing persisting sync tokens and crypto state in DB

example-config.yaml

@@ -138,6 +138,18 @@ bridge:
     # The prefix for commands. Only required in non-management rooms.
     command_prefix: "!wa"
 
+    # End-to-bridge encryption support options. This requires login_shared_secret to be configured
+    # in order to get a device for the bridge bot.
+    #
+    # Additionally, https://github.com/matrix-org/synapse/pull/5758 is required if using a normal
+    # application service.
+    encryption:
+        # Allow encryption, work in group chat rooms with e2ee enabled
+        allow: false
+        # Default to encryption, force-enable encryption in all portals the bridge creates
+        # This will cause the bridge bot to be in private chats for the encryption to work properly.
+        default: false
+
     # Permissions for using the bridge.
     # Permitted values:
     # relaybot - Talk through the relaybot (if enabled), no access otherwise