openchannel hook: add new close_to field

Rounds out the application of `upfront_shutdown_script`, allowing
an accepting node to specify a close_to address.

Prior to this, only the opening node could specify one.

Changelog-Added: Plugins: Allow the 'accepter' to specify an upfront_shutdown_script for a channel via a `close_to` field in the openchannel hook result

tests/plugins/accepter_close_to.py

@@ -0,0 +1,35 @@
+#!/usr/bin/env python3
+"""Simple plugin to test the openchannel_hook's
+   'close_to' address functionality.
+
+   If the funding amount is:
+      - a multiple of 11: we send back a valid address (regtest)
+      - a multiple of 7: we send back an empty address
+      - a multiple of 5: we send back an address for the wrong chain (mainnet)
+      - otherwise: we don't include the close_to
+"""
+
+from lightning import Plugin, Millisatoshi
+
+plugin = Plugin()
+
+
+@plugin.hook('openchannel')
+def on_openchannel(openchannel, plugin, **kwargs):
+    # - a multiple of 11: we send back a valid address (regtest)
+    if Millisatoshi(openchannel['funding_satoshis']).to_satoshi() % 11 == 0:
+        return {'result': 'continue', 'close_to': 'bcrt1q7gtnxmlaly9vklvmfj06amfdef3rtnrdazdsvw'}
+
+    # - a multiple of 7: we send back an empty address
+    if Millisatoshi(openchannel['funding_satoshis']).to_satoshi() % 7 == 0:
+        return {'result': 'continue', 'close_to': ''}
+
+    # - a multiple of 5: we send back an address for the wrong chain (mainnet)
+    if Millisatoshi(openchannel['funding_satoshis']).to_satoshi() % 5 == 0:
+        return {'result': 'continue', 'close_to': 'bc1qlq8srqnz64wgklmqvurv7qnr4rvtq2u96hhfg2'}
+
+    # - otherwise: we don't include the close_to
+    return {'result': 'continue'}
+
+
+plugin.run()