plugins: add feature_set to init object.

Shows what features we use in various contexts, including those added
by plugins in getmanifest.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Added: Plugin: `feature_set` object added to `init`

tests/plugins/show_feature_set.py

@@ -0,0 +1,17 @@
+#!/usr/bin/env python3
+from pyln.client import Plugin
+
+plugin = Plugin()
+
+
+@plugin.init()
+def init(options, configuration, plugin):
+    plugin.feature_set = configuration['feature_set']
+
+
+@plugin.method('getfeatureset')
+def getfeatureset(plugin):
+    return plugin.feature_set
+
+
+plugin.run()