nixos: nix.sshServe: add write option
nixos: nix.sshServe: add write option

Adds the ability to provide the --write flag in addition to the --serve flag via
a new option, services.sshServe.write.

A user can now share their system as a remote builder with friends easily as
follows:

{
  nix = {
    sshServe = {
      enable = true;
      write = true;
      keys = ["ssh-dss AAAAB3NzaC1k... alice@example.org"];
    };
  };
}

Co-authored-by: Raphael Megzari <ec67638a84dca5d1ffa2be696be98fba4d39bfb8@megzari.com>

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