The gRPC client unconditionally uses the caFile and certificateKeyFile for egress communication (https://github.com/10gen/mongo/blob/74903a8e3c2fc35a632ae6c3e246c3017980e2da/src/mongo/transport/grpc/grpc_transport_layer_impl.cpp#L196-L201), although the spec for outbound TLS communication state that the net.tls.clusterCAFile / net.tls.clusterFile certificates should be used for egress communication if they are available. We should fix this.