Add `createNewItemFromValue` option and `clearDropdown` method to `create_item_dropdown`

What does this MR do?

EE MR, https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4307

• Add createNewItemFromValue option to create_item_dropdown
  • Needed for dynamic variable list so we have * created with a title of All environments, https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4110
• Add clearDropdown method to create_item_dropdown
  • Needed for dynamic variable list to clear the cloned rows dropdown, https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4110

Environment scope dropdown

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4110

Are there points in the code the reviewer needs to double check?

The create_item_dropdown is used for the protected branch and tag dropdowns on the <namespace>/<project>/settings/repository page.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
  • No user-facing changes
• Tests added for this feature/bug
• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?