Numbers represented as Strings

[Zacholme7]

A bit of a nit, but there is a lot of mixing and matching between representing numbers as integers or strings. These should all just be integers

For example here, slot number should be an integer

[alan-ssvlabs]

These are because they come from the custom json marshaler of the phase0 types.

To convert them to int, we will create quite a few custom marshalers for the test types with auxiliary data structures. They are in different nested data structures. Personally, I think it adds complexity and is probably not so worthy just to convert them to int.

WDYT? @GalRogozinski