#21766 Added support for filter_by: alpha in templates system

It is now possible to constrain a key by `filter_by: alpha`. This means that the field value can be all *normal* characters but no digits. By normal characters we mean A-Za-z in anglosaxian countries, A-Öa-ö in Sweden, hiragana, katakana and kanji in Japan etc etc.

```
keys:
    name:
        type: str
        filter_by: alpha
```