silo_user_fetch_by_external_id should use LookupPath machinery

[jmpesp]

From #1210:

This seems really analogous to a lookup of, say, a Project by name within its parent Organization. It'd be nice to use LookupPath here, since it ensures consistent patterns, authz, filter conditions, etc. But I guess we can't do that now because this isn't actually a "name" field (different field name and type).

It would be nice to use

let (_, db_silo_user) = LookupPath::new(&opctx, datastore)
  .silo_name(some name)
  .silo_user_external_id(some external id)
  .fetch()
  .await?;

silo_user_fetch_by_external_id returns Result<Option<SiloUser>, Error> so the LookupPath code will have to add a function with an optional return value.