wireproto: expose the list of getbundle arguments to extensions

For now, getbundle accepts a fixed number of arguments: ``heads``, ``common``
and ``bundlecaps``. We make this list exposed at the module level to let
extensions add content there. This is important for extensions that wish to use
bundle2 for other contents than changegroup.

--HG--
branch : stable
extra : rebase_source : 8b3dc18e662e13b1fde39a422b82218d24dbbc7e