Bring in the dropdown component variant with search

Problem

The dropdown component variant with search was added in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8519. This is not a gitlab-ui component yet.

Proposal

Let's bring in the dropdown component variant with search as a gitlab-ui component, according to the specs.

See also the filter box (search) documentation.

If there are more than 10 elements in the list, it may be necessary to include a filter box.