Support custom target_user (#7)

[deric]

Support storing exported key under different account than it was generated from:

pubkey::ssh { 'alice_ed25519':
  target_user => 'bob',
  tags        => ['users'],
}

will read public ssh key from /home/alice/.ssh/id_ed25519.pub and store it to /home/bob/.ssh/authorized_keys on node where the resource is imported:

Ssh_authorized_key <<| tag == 'users' |>>