Improve context manager

#### I'm submitting a ...
- [x] feature request

#### What is the current behavior?

Context manager for `allure.step()` does not allow assigning a variable name, which would be useful.

#### What is the expected behavior?

I'd like it if this worked:
```py
with allure.step('foo') as s:
print(s.title)
```

I expected that to work because this does work:
```py
s = allure.step('foo')
print(s.title)
```

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

Having this named context manager would keep very simple code DRY.

#### Please tell us about your environment:

- Allure version: 🤷‍♂️ I'm just doing the code review, not sure how to determine this.
- Test framework: pytest@^8.2.2
- Allure adaptor: allure-pytest@^2.13.5

allure.step() のコンテキストマネージャーのエントリポイントから開始し、2つの使用例を pytest で再現してください。`with allure.step('foo') as s` が直接代入と同じ `title` 値を公開すれば変更は完了です。この動作に関する関連テストを追加または更新してください。

索引モデルが issue の本文から書いたものです。