TF-332 Add loadingState for update email drafts and get content email

lib/features/composer/domain/state/update_email_drafts_state.dart

@@ -1,6 +1,14 @@
 import 'package:core/core.dart';
 import 'package:jmap_dart_client/jmap/mail/email/email.dart';
 
+class UpdatingEmailDrafts extends UIState {
+
+  UpdatingEmailDrafts();
+
+  @override
+  List<Object?> get props => [];
+}
+
 class UpdateEmailDraftsSuccess extends UIState {
 
   final Email emailAsDrafts;

lib/features/composer/domain/usecases/update_email_drafts_interactor.dart

@@ -12,6 +12,7 @@ class UpdateEmailDraftsInteractor {
 
   Stream<Either<Failure, Success>> execute(AccountId accountId, Email newEmail, EmailId oldEmailId) async* {
     try {
+      yield Right<Failure, Success>(UpdatingEmailDrafts());
       final newEmailDrafts = await emailRepository.updateEmailDrafts(accountId, newEmail, oldEmailId);
       if (newEmailDrafts != null) {
         yield Right<Failure, Success>(UpdateEmailDraftsSuccess(newEmailDrafts));