openchannel2: add channel_max_msat to openchannel2 hook payload

Changelog-Added: Plugins: add a `channel_max_msat` value to the `openchannel2` hook. Tells you the total max funding this channel is allowed to have.

tests/test_plugin.py

@@ -606,6 +606,7 @@ def test_openchannel_hook(node_factory, bitcoind):
             'feerate_our_min': '1875',
             'locktime': '.*',
             'their_funding': '100000000msat',
+            'channel_max_msat': '16777215000msat',
         })
     else:
         expected.update({