HTTP API: allow querying users by external auth linked id

[hugodutka]

I propose adding a new external_auth_linked_id query filter to the /users endpoint, which would filter users by the linked_id field on the user_links table.

This would allow external integrations, like our coder/start-workspace-action, to look up the Coder user associated with an external account. The start workspace action currently filters users with the github_com_user_id field on the users table, which is incompatible with GitHub Enterprise.