Expose FixtureFunctionDefinition as part of the public API

There doesn’t appear to be a public type that can be used to annotate the result of `pytest.fixture`.

For example, consider a factory which creates fixtures:

```python
from _pytest.fixtures import FixtureFunctionDefinition

def make_fixture(value: str) -> FixtureFunctionDefinition:
    @pytest.fixture
    def fixture() -> str:
        return value
    return fixture
```