bake: prototype for composable bake attributes

[jsternberg]

This allows using either the csv syntax or a map to specify certain attributes.

[jsternberg]

Very incomplete first draft. This requires some changes to the gohcl package in the hcl package for it to work. The biggest thing is overriding the ImpliedType method and copying the full implementation from go-cty so that we can customize the behavior for certain types. In this case, I create a FromCtyValue interface and that works for our purposes.

The only changes to gohcl is creating a DecodeOptions which allows us to inject our own version of ImpliedType into the decoder. I also made it so we can change convert, but modifying the convert implementation was a lot more involved than modifying ImpliedType and using the existing capsule type.

[tonistiigi]

We shouldn't encode objects to csv string. That is backwards as the (typed) object is what we need.

If you want to reduce duplication then there can be generic csv -> map[k]v convert and then map to struct, although not sure if worth it.

[jsternberg]

Updated the prototype so the map[k]v version is the canonical form.

To reduce duplication, I've created a separate ExportEntry type with the same attributes. This type will implement encoding.TextUnmarshaler and json.Unmarshaler to control the marshaling and unmarshaling process in a consistent way. The type also has a ToPB() method to convert the type to the controller API version.

The csv parsing has been extracted into UnmarshalText and the original ParseExports method has been changed to be a loop that does this:

var out ExportEntry
if err := out.UnmarshalText([]byte(s)); err != nil {
    return nil, err
}
outs = append(outs, out.ToPB())

The a FromCtyValue method has been created so we can still detect it. But, we now only need one implementation that uses UnmarshalText and JSON unmarshaling and we should be able to reuse this implementation for each of these types.