add support for variables_from_script when subtype is multi

[rmaclean]

What resources or data sources are affected?

synthetics test

Feature Request

When using datadog_synthetics_test terraform resource with type as api and subtype is http I can specify a variables_from_script and it will work fine.

However when I change subtype to multi it fails to create with

"Additional properties are not allowed ('variablesFromScript' was unexpected)"

Please add support of variables_from_script to multistep

References

No response