pyln: add context manager to simpify filter use.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Added: pyln: LightningRpc has new `reply_filter` context manager for reducing output of RPC commands.

contrib/pyln-client/pyln/client/lightning.py

@@ -285,6 +285,7 @@ class UnixDomainSocketRpc(object):
         self.executor = executor
         self.logger = logger
         self._notify = None
+        self._filter = None
         if caller_name is None:
             self.caller_name = os.path.splitext(os.path.basename(sys.argv[0]))[0]
         else:
@@ -379,6 +380,8 @@ class UnixDomainSocketRpc(object):
             "id": this_id,
         }
 
+        if filter is None:
+            filter = self._filter
         if filter is not None:
             request["filter"] = filter
 
@@ -438,6 +441,22 @@ class UnixDomainSocketRpc(object):
         yield
         self._notify = old
 
+    @contextmanager
+    def reply_filter(self, filter):
+        """Filter the fields returned from am RPC call (or more than one)..
+
+        This is a context manager and should be used like this:
+
+        ```python
+        with rpc.reply_filter({"transactions": [{"outputs": [{"amount_msat": true, "type": true}]}]}):
+            rpc.listtransactions()
+        ```
+        """
+        old = self._filter
+        self._filter = filter
+        yield
+        self._filter = old
+
 
 class LightningRpc(UnixDomainSocketRpc):
     """