Add optional features definition

Add features structure for each zone.

---

Features are defined in defs.cue. It is possible to add a new feature definition like so:

#Features: {
    // ...
    my_feature?: {
        // params
        x: int
        y: bool
        z: string
    }
}

If the feature is only available for AWS zones, you should define it like this instead:

#AWSFeatures: {
    #Features
    // ...
    my_feature?: {
        // params
        x: int
        y: bool
        z: string
    }
}

You can enable the feature my_feature for a zone in this way:

zones: {
    my_zone: {
        // ...
        features: {
            my_feature: {
                x: 1
                y: true
                z: "blah"
            }
        }
    }
}