Make UseJSONNumber the default

- Remove the `UseJSONNumber` `ShowOption` added in https://github.com/hashicorp/terraform-exec/pull/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.