bundle2: support for capabilities with values The capabilities attributes of `bundle20` is now a dictionary and the reply caps can encode capabilities with values.

bundle2: add capabilities support in `replycaps` part This part now contains a list of supported capabilities.

bundle2: adds a capabilities attribute on bundler20 This attribute conveys the capabilities supported by the destination of the bundle. It is used to decide which parts to include in the bundle. This is currently a set but will probably be turned into a dictionary to allow capabilities with values.