HTTP API: allow querying users by external auth linked id

I propose adding a new `external_auth_linked_id` query filter to the [/users endpoint](https://github.com/coder/coder/blob/c1b2304d183aa73213f057d9531f7a0d357e9d30/coderd/users.go#L248), which would filter users by the `linked_id` field on the [user_links table](https://github.com/coder/coder/blob/c1b2304d183aa73213f057d9531f7a0d357e9d30/coderd/database/dump.sql#L1775).

This would allow external integrations, like our [coder/start-workspace-action](https://github.com/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.