Store mailbox cache by AccountId

(cherry picked from commit 7804f19d27d65af855e145de1953df9a553f3ce0)

model/lib/extensions/account_id_extensions.dart

@@ -0,0 +1,6 @@
+
+import 'package:jmap_dart_client/jmap/account_id.dart';
+
+extension AccountIdExtension on AccountId {
+  String get asString => id.value;
+}

model/lib/extensions/email_id_extensions.dart

@@ -0,0 +1,5 @@
+import 'package:jmap_dart_client/jmap/mail/email/email.dart';
+
+extension EmailIdExtension on EmailId {
+  String get asString => id.value;
+}

model/lib/extensions/mailbox_id_extension.dart

@@ -23,4 +23,6 @@ extension MailboxIdExtension on MailboxId {
       generatePath():  true,
     });
   }
+
+  String get asString => id.value;
 }