Allow TwoPartyClient to define a bootstrap interface

c++/src/capnp/rpc-twoparty.c++

@@ -179,6 +179,12 @@ TwoPartyClient::TwoPartyClient(kj::AsyncIoStream& connection)
     : network(connection, rpc::twoparty::Side::CLIENT),
       rpcSystem(makeRpcClient(network)) {}
 
+
+TwoPartyClient::TwoPartyClient(kj::AsyncIoStream& connection,
+                               Capability::Client bootstrapInterface)
+    : network(connection, rpc::twoparty::Side::CLIENT),
+      rpcSystem(network, bootstrapInterface) {}
+
 Capability::Client TwoPartyClient::bootstrap() {
   MallocMessageBuilder message(4);
   auto vatId = message.getRoot<rpc::twoparty::VatId>();

c++/src/capnp/rpc-twoparty.h

@@ -134,6 +134,7 @@ class TwoPartyClient {
 
 public:
   explicit TwoPartyClient(kj::AsyncIoStream& connection);
+  TwoPartyClient(kj::AsyncIoStream& connection, Capability::Client bootstrapInterface);
 
   Capability::Client bootstrap();
   // Get the server's bootstrap interface.