Store the impersonator id on access tokens created via ZED_IMPERSONATE (#4108)

* Use the impersonator id to prevent these tokens from counting against
the impersonated user when limiting the users' total of access tokens.
* When connecting using an access token with an impersonator add the
impersonator as a field to the tracing span that wraps the task for that
connection.
* Disallow impersonating users via the admin API token in production,
because when using the admin API token, we aren't able to identify the
impersonator.

crates/collab/src/tests/test_server.rs

@@ -213,6 +213,7 @@ impl TestServer {
                                 server_conn,
                                 client_name,
                                 user,
+                                None,
                                 Some(connection_id_tx),
                                 Executor::Deterministic(cx.background_executor().clone()),
                             ))