push: move bundle2-pushkey based bookmarks exchange in its own function We are about to introduce an alternative way to push bookmark over bundle2.

bookmark: add pushkey hook compatiblity to the bundle2 part Currently, pushing a bookmark update triggers a pushkey hooks. It is likely that users in the wild use such hooks to control bookmark movement. Using a non push-key mechanism to exchange bookmark means these hooks are no longer called, possibly breaking existing users setup. So we add explicit call to the pushkey hooks in the handling of the bundle2 part. This behavior can be disabled with a new config knob: 'server.bookmarks-pushkey-compat'.

bookmark: introduce a 'bookmarks' part This part can carry and apply bookmarks information. We start with adding the core behavior of the part. In its current form, the part is only suitable for push since it plain update the bookmark without consideration for the local state. Support of the behavior needed for pulling will be added in later changesets.