This adds constraints for asserting that a given attribute or value
exists, or does not exist. This acts as a companion to =, or !=
operators, e.g:

```hcl
constraint {
        attribute = "${attrs.type}"
        operator  = "!="
        value     = "database"
}

constraint {
        attribute = "${attrs.type}"
        operator  = "is_set"
}
```