NetworkInterfaceTL's connection pool was designed around server-to-server authentication: when a pooled connection is set up, it authenticates using the node's internal credentials (X.509 or keyfile). There is no supported path for a caller to supply external user credentials and have those credentials applied during pool connection setup. This change plumbs in support for providing those credentials, so we can use the NetworkInterfaceTL as a backend for async clients in jstests.