Add config option to turn on in-room event sending timing metrics

This is intended to be hooked up to an external system. 

Due to the extra events and metadata concerns, this is only available if turned on from the config.

See `sendTimePerformanceMetrics.ts` for event schemas.

src/settings/Settings.tsx

@@ -759,6 +759,10 @@ export const SETTINGS: {[setting: string]: ISetting} = {
         default: true,
         controller: new ReducedMotionController(),
     },
+    "Performance.addSendMessageTimingMetadata": {
+        supportedLevels: [SettingLevel.CONFIG],
+        default: false,
+    },
     "Widgets.pinned": { // deprecated
         supportedLevels: LEVELS_ROOM_OR_ACCOUNT,
         default: {},