Device manager - last_seen_user_agent device property (#2728)

* add last_seen_user_agent to IMyDevice type

* add ubstable value

* use unstable value in interface

Author: Kerry

src/client.ts

@@ -215,6 +215,11 @@ export const CRYPTO_ENABLED: boolean = isCryptoAvailable();
 const CAPABILITIES_CACHE_MS = 21600000; // 6 hours - an arbitrary value
 const TURN_CHECK_INTERVAL = 10 * 60 * 1000; // poll for turn credentials every 10 minutes
 
+export const UNSTABLE_MSC3852_LAST_SEEN_UA = new UnstableValue(
+    "last_seen_user_agent",
+    "org.matrix.msc3852.last_seen_user_agent",
+);
+
 interface IExportedDevice {
     olmDevice: IExportedOlmDevice;
     userId: string;
@@ -688,6 +693,8 @@ export interface IMyDevice {
     display_name?: string;
     last_seen_ip?: string;
     last_seen_ts?: number;
+    [UNSTABLE_MSC3852_LAST_SEEN_UA.stable]?: string;
+    [UNSTABLE_MSC3852_LAST_SEEN_UA.unstable]?: string;
 }
 
 export interface IDownloadKeyResult {