Remove TS overloads in favor of unions / simpler signatures

[lakardion]

The main approach for the surface type layer of the library is based on function overloads. This approach helped with type inference of the different ways users could call the functions. However, overloads make the TS code really hard to read and maintain.

Although this might not be the ultimate solution (since TS library is a bit of a pain) it may greatly improve the ease for maintenance and change.
Considering microsoft/TypeScript#43371 , we might be able to shift towards a single function (and single input parameter) signature and get all the inferences right from a single parameter rather than having to define multiple overloads to tell apart the different use cases and types of the functions.
I believe this issue was also the reason of why we were requiring two parameters in createCustomServiceCall to be able to infer the callback type from the passed models.

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