erl_tar: Extend api to define own storage media access functions.

The reason for this is a requirement on enabling ssh_sftp to write a tar file on the server.
This new api function is used by ssh_sftp:open_tar/3,4.
ssh: Add ssh_sftp:open_tar/3,4 which opens a tar file at the server side of a channel.

Depends on erl_tar.erl having the function erl_tar:init/3 defined.
ssh: update vsn.mk
Merge branch 'hans/ssh/erl_tar_sftp/OTP-12180' into maint

* hans/ssh/erl_tar_sftp/OTP-12180:
  ssh: update vsn.mk
  ssh: Add ssh_sftp:open_tar/3,4 which opens a tar file at the server side of a channel.
  erl_tar: Extend api to define own storage media access functions.