refactor: make return type of `BaseDiffHandler.CalculateCurrentAndTargetIndexes` understandable

[moreal]

Context

Currently, the return type of BaseDiffHandler.CalculateCurrentAndTargetIndexes is a tuple of four long elements, (long, long, long, long).

https://github.com/planetarium/mimir/blob/822a0c3e92d5c68c777bdc43168f589e8e668c72/Mimir.Worker/Handler/BaseDiffHandler.cs#L69

When using the method, I cannot know the role of each element without looking inside the method. I think it can be more understandable by the following ways:

• Document the method to describe return type.
• Use named-tuple.
• Define class or record to describe the return type and use the type.

[Mecit-SA]

Could you assign this to me, please? :)

[Atralupus]

Could you assign this to me, please? :)

Sure!