builtin/providers/terraform: terraform_ssh_tunnels ephemeral resource…

… type

This is here only for the purposes of prototyping ephemeral resources. If
we move forward with a "real" implementation then something like this would
be better placed in a separate SSH provider, rather than built into
Terraform CLI itself.

This is just a basic implementation to get started with. It's probably
not very robust and will probably need fixes and additions in future
commits.