Support multiple integration managers behind a labs flag

Fixes https://github.com/vector-im/riot-web/issues/10622
Implements [MSC1957](https://github.com/matrix-org/matrix-doc/pull/1957)

Design is not final.

src/settings/Settings.js

@@ -121,6 +121,12 @@ export const SETTINGS = {
         supportedLevels: LEVELS_FEATURE,
         default: false,
     },
+    "feature_many_integration_managers": {
+        isFeature: true,
+        displayName: _td("Multiple integration managers"),
+        supportedLevels: LEVELS_FEATURE,
+        default: false,
+    },
     "MessageComposerInput.suggestEmoji": {
         supportedLevels: LEVELS_ACCOUNT_SETTINGS,
         displayName: _td('Enable Emoji suggestions while typing'),