Incorrect type of `start` and `stop` keys in `StartStopEntry`

[yousefmoazzam]

The values of the start and stop keys have the following constraints:

1. can be omitted
2. can be None
3. can be an int
4. can be any of the following "keywords": begin, mid, and end

The current type for them is Union[int, str]:

httomo/httomo/transform_loader_params.py

Lines 34 to 42 in 584e15c

	class StartStopEntry(TypedDict):
	"""
	Configuration for a single dimension's previewing in terms of start/stop values.
	"""
	start: Union[int, str]
	start_offset: Optional[int]
	stop: Union[int, str]
	stop_offset: Optional[int]

which doesn't reflect all those constraints: it misses 1 and 2, and doesn't constraint the possible strings to be only the three available in 4.

I think the correct type would be NotRequired[Optional[Literal["begin"] | Literal["mid"] | Literal["end"]]].