Feature/istwcms 6134 ocardena field attributtes

Summary of changes

Add webform fields attributes on the migration source.

How to test?

1. Checkout this branch
2. run drush cr && drush mr uw_webform && drush mim uw_webform
3. Go /admin/structure/webform/manage/webform_3 and verify the date field migrated with the minimum and maximum values.
4. Go admin/structure/webform/manage/webform_233 and verify the 8_character_userid field was migrated with the minlength and maxlength values.
5. Check the webform fields attributes are migrating well.