Add type hints for better mypy support

Hello!

We use mypy to type check our pytest tests. So we have to use wrappers for allure helpers, e.g.

```
from collections.abc import Callable
from typing import Any, TypeVar, cast

import allure

F = TypeVar('F', bound=Callable[..., Any])

def epic(*items: str) -> Callable[[F], F]:
return cast('Callable[[F], F]', allure.epic(*items)) # type: ignore[no-untyped-call]

def tag(*tags: str) -> Callable[[F], F]:
return cast('Callable[[F], F]', allure.tag(*tags)) # type: ignore[no-untyped-call]

... and so on
```
or even use global mute for untyped calls in typed context.

Can you please clarify if there are any plans to introduce type hints into framework?

Thank you.