[Feature request] Make `StepContext` available in public API for annotations

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

#### What is the current behavior?
```python
def allure_step__foo(content: str):
return allure.step(content)
```
Annotating return type of such function is not possible, because `allure.step` returns internal `StepContext` (under `_allure.py`)

#### What is the expected behavior?

Make `StepContext` available in `allure.py`

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

Adding `StepContext` to public API will allow to annotate return type of `allure_step__foo` in a nice and simple way via:

```python
def allure_step__foo(content: str) -> StepContext: ...
```