lightningd: print out support for channel_types in --list-features.

It's not a separate option, but lnprototest needs it to know to expect
the tlvs.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/test_misc.py

@@ -1924,6 +1924,7 @@ def test_list_features_only(node_factory):
         expected += ['option_anchor_outputs/odd']
         expected += ['option_shutdown_anysegwit/odd']
         expected += ['option_onion_messages/odd']
+        expected += ['supports_open_accept_channel_type']
     else:
         expected += ['option_shutdown_anysegwit/odd']
     assert features == expected