Fix validation of engine type

[patryk-przekwas-airspace-intelligence]

When explicitly setting the “classic” instance engine value for a Pulsar instance, Terraform fails with the following validation error:

│ Error: "engine" must be pulsar
│ 
│   with streamnative_pulsar_instance.asi_dev,
│   on main.tf line 69, in resource "streamnative_pulsar_instance" "asi_dev":
│   69:   engine            = "pulsar"
│ 

[maxsxu]

Thanks for your contribution!

[tuteng]

It seems to have some tests that non SN members can't pass. We need to ask an administrator how to enable non SN members to contribute to it. Would you like me to create a new pull request? @patryk-przekwas-airspace-intelligence cc @freeznet @maxsxu