feat: Add a way to get a verification request by its transaction id

A client might find the need to get the verification request object by
its transaction id, to be able to e.g. display for in-room verification
an "accept verification request" button easily.

lib/encryption/key_verification_manager.dart

@@ -52,6 +52,8 @@ class KeyVerificationManager {
     _requests[request.transactionId] = request;
   }
 
+  KeyVerification getRequest(String requestId) => _requests[requestId];
+
   Future<void> handleToDeviceEvent(ToDeviceEvent event) async {
     if (!event.type.startsWith('m.key.verification.') ||
         client.verificationMethods.isEmpty) {

test/encryption/key_verification_test.dart

@@ -128,6 +128,11 @@ void main() {
       await sub.cancel();
       expect(req2 != null, true);
 
+      expect(
+          client2.encryption.keyVerificationManager
+              .getRequest(req2.transactionId),
+          req2);
+
       // send ready
       FakeMatrixApi.calledEndpoints.clear();
       await req2.acceptVerification();