support import interface

[joeyh]

This is a new interface for special remotes that will ship in the next release of git-annex. It allows git-annex import from an external special remote, which expands the universe of potential special remotes significantly.

https://git-annex.branchable.com/design/external_special_remote_protocol/export_and_import_appendix/#index2h2

https://git-annex.branchable.com/tips/how_to_make_a_simple_importtree_special_remote/