Support searching for users using GraphQL

Problem to solve

It's not currently possible to search across users in a project via GraphQL. This functionality is required for a number of upcoming features; including user mapping in the Jira Importer. We should build this so that it's possible to suggest users using a widget similar to the autocomplete endpoint (https://gitlab.com/-/autocomplete/users.json) used by the sidebar assignees feature.

See #220527 (comment 372624011)

Proposal

Further details

Permissions and Security

Documentation

Requires a documentation update to the GraphQL reference.

Availability & Testing

What does success look like, and how can we measure that?

A performant, functional user search in GraphQL that can work with partial strings.

Links / references