Backpacks should support multiple pages for even bigger backpacks.

There should be an option to create Size configs in the config file like:

Size:
  Mega:
    Pages: 3
    SlotsPerPage: 50
    BlockedSlotsMaterial: Barrier
  Slots213:
    Pages: auto
    SlotsPerPage: auto
    Slots: 213
    BlockedSlotsMaterial: Barrier

The permission to use backpacks with the size from the config should be backpack.size.<name in config (lowercase)>, for example: the Mega config should use the permission backpack.size.mega.

In the backpack there should be navigation buttons (arrow) in the last 2 slots of the current page.
It also should be possible to open a certain page directly with a command like /backpack page <page number>.