Extend keys with enums

The goal is to shorten repeative definitions like

```yaml
My\App\Entity\Currency:
    currency_eur:
        isoCode: eur
    currency_usd:
        isoCode: usd
    currency_chf:
        isoCode: chf
    currency_dkk:
        isoCode: dkk
```

... with some enum functionality in keys

```yaml
My\App\Entity\Currency:
    currency_{eur,usd,chf,dkk}:
        isoCode: <current()>
```

... that keeps syntax declarative.

Maybe the syntax I used is not good, we need to discuss about it.