api: use cleanhttp.DefaultPooledTransport for default API client (#12492)

We expect every Nomad API client to use a single connection to any
given agent, so take advantage of keep-alive by switching the default
transport to `DefaultPooledClient`. Provide a facility to close idle
connections for testing purposes.

Restores the previously reverted #12409


Co-authored-by: Ben Buzbee <bbuzbee@cloudflare.com>

command/agent/http_test.go

@@ -1113,6 +1113,8 @@ func TestHTTPServer_Limits_OK(t *testing.T) {
 			conf.Address = a.HTTPAddr()
 			conf.TLSConfig.Insecure = true
 			client, err := api.NewClient(conf)
+			defer client.Close()
+
 			require.NoError(t, err)
 
 			// Assert a blocking query isn't timed out by the