Verify Element X with an existing Element Classic account. (#5374)

* Read and import the secrets from ClassicAppAccounts.

* Record snapshots.

* Add some documentation, tidy up tests and fix the dismissal of the backup instructions.

* Workaround flakey tests (the fulfilments weren't always firing).

* Allow a custom Classic App deep link URL to be configured.

ElementX/Sources/Services/Authentication/AuthenticationClientFactory.swift

@@ -17,6 +17,11 @@ protocol AuthenticationClientFactoryProtocol {
                     clientSessionDelegate: ClientSessionDelegate,
                     appSettings: AppSettings,
                     appHooks: AppHooks) async throws -> ClientProtocol
+    
+    func makeInMemoryClient(homeserverAddress: String,
+                            clientSessionDelegate: ClientSessionDelegate,
+                            appSettings: AppSettings,
+                            appHooks: AppHooks) async throws -> ClientProtocol
 }
 
 /// A wrapper around `ClientBuilder` to allow for mocked clients to be injected into authentication tests.
@@ -40,4 +45,21 @@ struct AuthenticationClientFactory: AuthenticationClientFactoryProtocol {
             .serverNameOrHomeserverUrl(serverNameOrUrl: homeserverAddress)
             .build()
     }
+    
+    func makeInMemoryClient(homeserverAddress: String,
+                            clientSessionDelegate: ClientSessionDelegate,
+                            appSettings: AppSettings,
+                            appHooks: AppHooks) async throws -> ClientProtocol {
+        try await ClientBuilder
+            .baseBuilder(httpProxy: appSettings.websiteURL.globalProxy,
+                         slidingSync: .discover,
+                         sessionDelegate: clientSessionDelegate,
+                         appHooks: appHooks,
+                         enableOnlySignedDeviceIsolationMode: appSettings.enableOnlySignedDeviceIsolationMode,
+                         enableKeyShareOnInvite: appSettings.enableKeyShareOnInvite,
+                         threadsEnabled: appSettings.threadsEnabled)
+            .inMemoryStore()
+            .serverNameOrHomeserverUrl(serverNameOrUrl: homeserverAddress)
+            .build()
+    }
 }