Add plugin notification topic "block_processed".

Changelog-Added: Plugins: Added notification topic "block_processed".

tests/plugins/block_processed.py

@@ -0,0 +1,22 @@
+#!/usr/bin/env python3
+
+from pyln.client import Plugin
+
+
+plugin = Plugin()
+
+blocks_catched = []
+
+
+@plugin.subscribe("block_processed")
+def notify_block_processed(plugin, block, **kwargs):
+    global blocks_catched
+    blocks_catched.append(block["height"])
+
+
+@plugin.method("blockscatched")
+def return_moves(plugin):
+    return blocks_catched
+
+
+plugin.run()