Improve typing for WithXPermissions fixtures

The types defined [here](https://github.com/Flagsmith/flagsmith/blob/main/api/tests/types.py#L7-L15) are not valid according to mypy. Currently, all usages of the functions returned by these fixtures raise the following error in mypy's type checking:

```
error: Too few arguments [call-arg]
```

I have already tried moving to using a Protocol definition like the following:

```
class WithEnvironmentPermissionsCallable(Protocol):
def __call__(
self,
permission_keys: Optional[list[str]] = None,
environment_id: Optional[int] = None,
admin: bool = False,
) -> UserEnvironmentPermission: ...
```

However, this just results in a different error:

```
error: Incompatible return value type (got "Callable[[list[str] | None, int | None, bool], UserEnvironmentPermission]", expected "WithEnvironmentPermissionsCallable") [return-value]
note: "WithEnvironmentPermissionsCallable.__call__" has type "Callable[[DefaultArg(list[str] | None, 'permission_keys'), DefaultArg(int | None, 'environment_id'), DefaultArg(bool | None, 'admin')], UserEnvironmentPermission]"
```