Adds create button to the search dropdown

What does this MR do?

Adds create button to the search dropdown to allow for https://gitlab.com/gitlab-org/gitlab-ee/issues/8621

when there are no results, we will show a button that allows creating a new entry with the value typed in the search box

We have this behaviour currently outside Vue only for the Variables list in the CI/CD settings page

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/8621

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer