FE - User Contribution Mapping - filter for placeholder users in the admin view
### Problem
Currently, in the Admin view for instance members, placeholder users are shown under the `Users` tab and don't have their own `Placeholders` tab.
This was noticed in our [test session](https://gitlab.com/gitlab-org/foundations/import-and-integrate/discussions/-/issues/112#note_2077660851) and just recently was reported as confusing behavior from a customer.
Update: It has been decided to only add a filter that shows the list of placeholders. See [this comment](https://gitlab.com/gitlab-org/gitlab/-/issues/521974#note_2407312470)

### Proposal (Updated)
Add filter in the UI so that Admin can filter for type=placeholder users in the filter bar.
~~Add filter in the UI so that Admin can filter placeholders out. By default the filter should be on, meaning by default the placeholders are filtered out.~~
issue