Unmarshal Custom Types

Is there a way to decode custom types? I'm looking for something equivalent to `encoding/json.Unmarshaler`. I've tried implementing `encoding.TextUnmarshaler` which didn't work.

Here's an example use-case.
``` go
type Duration struct {
time.Duration
}

func (d *Duration) UnmarshalText(data []byte) error {
d0, err := time.ParseDuration(string(data))
if err != nil {
return err
}
d.Duration = d0
return nil
}
```

SO Question: https://stackoverflow.com/questions/60515131/hcl-unmarshal-custom-types

Start by inspecting the repository’s existing Go decoding behavior and how it handles encoding.TextUnmarshaler; the issue provides no file or test path. Define the supported custom-type contract using the Duration example, then add coverage demonstrating successful decoding and error handling.

Written by the indexing model from the issue text.