Force cast features label values to string

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#### I'm submitting a ...
- feature request

#### What is the current behavior?
https://github.com/allure-framework/allure-python/blob/50cb0ec0f388928eabf1fc5af92a009851d34965/allure-python-commons/src/_allure.py#L45-L46

Features values set as is.

#### What is the expected behavior?

Cast `features` to string

#### What is the motivation / use case for changing the behavior?

For easily cast to feature name from dataclasses

Its look better
```python
@allure.feature(Features.some_feature1, Features.some_feature2)
```
vs
```python
@allure.feature(str(Features.some_feature1), str(Features.some_feature2))
```

#### Please tell us about your environment:

- Allure version: 2.1.0
- Test framework: pytest@6.2.4
- Allure adaptor: allure-pytest@2.9.43

#### Other information

[//]: # (
. e.g. detailed explanation, stacktraces, related issues, suggestions
. how to fix, links for us to have more context, eg. Stackoverflow, Gitter etc
)

Inizia da allure-python-commons/src/_allure.py intorno alle righe 45-46, dove i feature values vengono attualmente mantenuti così come forniti. Segui l’entry point del feature decorator e i suoi test esistenti, quindi assicurati che i feature values vengano convertiti in stringhe, così che i dataclass members possano essere passati direttamente; il lavoro è completato quando i feature names risultanti sono valori stringa.

Scritto dal modello di indicizzazione a partire dal testo della issue.