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
)

Bắt đầu tại allure-python-commons/src/_allure.py, khoảng dòng 45-46, nơi feature values hiện được giữ nguyên như được cung cấp. Theo dõi entry point của feature decorator và các bài kiểm thử hiện có của nó, sau đó đảm bảo feature values được chuyển thành chuỗi để có thể truyền trực tiếp dataclass members; hoàn thành khi các feature names tạo ra là các giá trị string.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.