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
)

Commencez dans allure-python-commons/src/_allure.py autour des lignes 45-46, où les feature values sont actuellement conservées telles qu’elles ont été fournies. Suivez le point d’entrée du feature decorator et ses tests existants, puis assurez-vous que les feature values sont converties en chaînes afin que les dataclass members puissent être passés directement ; le travail est terminé lorsque les feature names obtenus sont des valeurs de type string.

Rédigé par le modèle d'indexation à partir du texte de l'issue.