Add GraphQL project users autocomplete

What does this MR do and why?

This is equivalent to the AutocompleteController#users endpoint.

This will replace the projectMembers query that we currently use in app/assets/javascripts/issues/list/queries/search_users.query.graphql

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #387603 (closed)