pyln-client: Add facility to register featurebits from plugins

we have 4 venues in which we can add features, 3 of which are unilaterally
controlled (`init`, `node_announcement`, and `invoices`) the
`channel_announcement` is co-signed by both parties, so we can't add
featurebits without additional coordination overhead.

Each location is encoded as a key-value pair in a dict called `featurebits` in
the manifest (omitted if no custom featurebits are set).

contrib/pyln-client/pyln/client/plugin.py

@@ -1,3 +1,4 @@
+from binascii import hexlify
 from collections import OrderedDict
 from enum import Enum
 from .lightning import LightningRpc, Millisatoshi
@@ -5,6 +6,7 @@ from threading import RLock
 
 import inspect
 import json
+import math
 import os
 import re
 import sys
@@ -99,10 +101,37 @@ class Plugin(object):
 
     """
 
-    def __init__(self, stdout=None, stdin=None, autopatch=True, dynamic=True):
+    def __init__(self, stdout=None, stdin=None, autopatch=True, dynamic=True,
+                 init_features=None, node_features=None, invoice_features=None):
         self.methods = {'init': Method('init', self._init, MethodType.RPCMETHOD)}
         self.options = {}
 
+        def convert_featurebits(bits):
+            """Convert the featurebits into the bytes required to hexencode.
+            """
+            if bits is None:
+                return None
+
+            elif isinstance(bits, int):
+                bitlen = math.ceil(math.log(bits, 256))
+                return hexlify(bits.to_bytes(bitlen, 'big')).decode('ASCII')
+
+            elif isinstance(bits, str):
+                # Assume this is already hex encoded
+                return bits
+
+            elif isinstance(bits, bytes):
+                return hexlify(bits).decode('ASCII')
+
+            else:
+                raise ValueError("Could not convert featurebits to hex-encoded string")
+
+        self.featurebits = {
+            'init': convert_featurebits(init_features),
+            'node': convert_featurebits(node_features),
+            'invoice': convert_featurebits(invoice_features),
+        }
+
         # A dict from topics to handler functions
         self.subscriptions = {}
 
@@ -523,14 +552,21 @@ class Plugin(object):
             if method.long_desc:
                 methods[len(methods) - 1]["long_description"] = method.long_desc
 
-        return {
+        manifest = {
             'options': list(self.options.values()),
             'rpcmethods': methods,
             'subscriptions': list(self.subscriptions.keys()),
             'hooks': hooks,
-            'dynamic': self.dynamic
+            'dynamic': self.dynamic,
         }
 
+        # Compact the features a bit, not important.
+        features = {k: v for k, v in self.featurebits.items() if v is not None}
+        if features is not None:
+            manifest['featurebits'] = features
+
+        return manifest
+
     def _init(self, options, configuration, request):
         self.rpc_filename = configuration['rpc-file']
         self.lightning_dir = configuration['lightning-dir']