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
)

Beginne bei allure-python-commons/src/_allure.py ungefähr in den Zeilen 45–46, wo feature values derzeit so beibehalten werden, wie sie übergeben wurden. Verfolge den Einstiegspunkt des feature decorators und seine bestehenden Tests und stelle dann sicher, dass feature values in Strings umgewandelt werden, damit dataclass members direkt übergeben werden können; fertig ist die Änderung, wenn die resultierenden feature names Stringwerte sind.

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