Only offer to verify if a cross-signed device is available and improve the UX whilst waiting. (#4710)

* Only offer to verify if a cross-signed device is available

* Wait until we know which verification options are available before showing them.

---------

Co-authored-by: Hubert Chathi <hubertc@matrix.org>

ElementX/Sources/Mocks/Generated/GeneratedMocks.swift

@@ -2287,6 +2287,70 @@ class ClientProxyMock: ClientProxyProtocol, @unchecked Sendable {
             return isOnlyDeviceLeftReturnValue
         }
     }
+    //MARK: - hasDevicesToVerifyAgainst
+
+    var hasDevicesToVerifyAgainstUnderlyingCallsCount = 0
+    var hasDevicesToVerifyAgainstCallsCount: Int {
+        get {
+            if Thread.isMainThread {
+                return hasDevicesToVerifyAgainstUnderlyingCallsCount
+            } else {
+                var returnValue: Int? = nil
+                DispatchQueue.main.sync {
+                    returnValue = hasDevicesToVerifyAgainstUnderlyingCallsCount
+                }
+
+                return returnValue!
+            }
+        }
+        set {
+            if Thread.isMainThread {
+                hasDevicesToVerifyAgainstUnderlyingCallsCount = newValue
+            } else {
+                DispatchQueue.main.sync {
+                    hasDevicesToVerifyAgainstUnderlyingCallsCount = newValue
+                }
+            }
+        }
+    }
+    var hasDevicesToVerifyAgainstCalled: Bool {
+        return hasDevicesToVerifyAgainstCallsCount > 0
+    }
+
+    var hasDevicesToVerifyAgainstUnderlyingReturnValue: Result<Bool, ClientProxyError>!
+    var hasDevicesToVerifyAgainstReturnValue: Result<Bool, ClientProxyError>! {
+        get {
+            if Thread.isMainThread {
+                return hasDevicesToVerifyAgainstUnderlyingReturnValue
+            } else {
+                var returnValue: Result<Bool, ClientProxyError>? = nil
+                DispatchQueue.main.sync {
+                    returnValue = hasDevicesToVerifyAgainstUnderlyingReturnValue
+                }
+
+                return returnValue!
+            }
+        }
+        set {
+            if Thread.isMainThread {
+                hasDevicesToVerifyAgainstUnderlyingReturnValue = newValue
+            } else {
+                DispatchQueue.main.sync {
+                    hasDevicesToVerifyAgainstUnderlyingReturnValue = newValue
+                }
+            }
+        }
+    }
+    var hasDevicesToVerifyAgainstClosure: (() async -> Result<Bool, ClientProxyError>)?
+
+    func hasDevicesToVerifyAgainst() async -> Result<Bool, ClientProxyError> {
+        hasDevicesToVerifyAgainstCallsCount += 1
+        if let hasDevicesToVerifyAgainstClosure = hasDevicesToVerifyAgainstClosure {
+            return await hasDevicesToVerifyAgainstClosure()
+        } else {
+            return hasDevicesToVerifyAgainstReturnValue
+        }
+    }
     //MARK: - startSync
 
     var startSyncUnderlyingCallsCount = 0