Incorrect type of key in `StartStopEntry` for representing a key that may not be present

Same as #608, but for the `start_offset` and `stop_offset` keys in `StartStopEntry`: https://github.com/DiamondLightSource/httomo/blob/584e15ccd84fbd7f440454b6d3c48a071b8f2f83/httomo/transform_loader_params.py#L34-L42

`Optional[int]` should be replaced with `NotRequired[Optional[int]]` to correctly reflect the state of the dict that can be received from the start/stop config for a dimension in a `preview` parameter value.