Make UseJSONNumber the default

[kmoe]

• Remove the UseJSONNumber ShowOption added in Add ShowOption for configuring JSON decoding #427
• Call UseJSONNumber() method wherever a terraform-json State or Plan struct is used within terraform-exec to force use of json.Number for ints

This is a breaking change, as it switches the returned terraform-json types from float64 to json.Number for number representation, so any tfexec consumer that goes deep enough into the plan or state to read a number value will have a compile error.