Add support for SSH Key management

[jimeh]

Some of the resources below will require support within go-katapult, not to
mention additions to the Katapult Core API itself. As such this is issue is
marked as On-Hold for now.

• Add katapult_ssh_key resource with:
  • name attribute
  • public_key attribute
• Add katapult_ssh_key data source.
• Add authorized_keys attribute to katapult_virtual_machine resource
  with:
  • all_organization_keys bool attribute, for adding all organization SSH
    keys to the VM. Default value: true
  • all_user_keys bool attribute, for adding all keys for all users who
    are a member of the organization. Default value: true
  • keys attribute, list of katapult_ssk_key ID strings. When not empty
    set both all_organization_keys and all_user_keys to false.
  • users attribute, list of katapult_user ID strings. When not empty
    set both all_organization_keys and all_user_keys to false.
• Add katapult_user data source.
• Add katapult_users data source.