bundle2: transmit capabilities to getbundle during pull Bundle2 capabilities of the client are sent to the server in the bundlecaps argument of `getbundle`.

bundle2: include client capabilities in the pushed bundle The necessary data is now included in the `replycaps` part.

bundle2: advertise bundle2 caps in server capabilities We can now retrieve them from the server during push. The capabilities are encoded the same way as in `replycaps` part (with an extra layer of urlquoting to escape separators).

bundle2: add bundle2caps dict on localrepo object This dictionary will hold bundle2-related capabilities.

bundle2: capabilities encoding

bundle2: extract capabilities decoding We'll need to reuse this in more places (at least pull and push).

bundle2: protect capabilities name and values with url quoting This lift limitations of the text based encoding.

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.