enable step bookmark support (depends on OpenZFS Bookmark Copying) #326

• implement bookmark copying & ZCP bookmark support openzfs/zfs#9571 implements bookmark copying
• this provides the functionality required to create step bookmarks
• step bookmarks are zrepl's equivalent for zfs holds for bookmarks
• search for occurences of ErrBookmarkCloningNotSupported in the code to get started, most of the infrastructure is already there, it's just the zfs layer that needs
  • feature detection
  • invoke the command for bookmarks
  • platform test in platformtest/tests

Then

• remove special-case handling in package endpoint
• update docs/configuration/overview.rst
• test it