feat: add validation checks when using existing KSA

[loremaps]

See #1065

This PR adds checks for cluster_name and location variables, when using an existing KSA.

The error message is not great:

╷
│ Error: Invalid function argument
│
│   on ../../../../../personal/terraform-google-kubernetes-engine/modules/workload-identity/main.tf line 33, in locals:
│   33:   validate_cluster = (var.use_existing_k8s_sa && var.cluster_name == "") ? tobool("Variable 'cluster_name' is required when using existing KSA") : true
│     ├────────────────
│     │ while calling tobool(v)
│
│ Invalid value for "v" parameter: cannot convert "Variable 'cluster_name' is required when using existing KSA" to bool; only the strings "true" or "false" are allowed.
╵

But I was bitten by this issue two times already. I believe that this does give some guidance. But if you feel the error is too misleading, please feel free to close this PR 😅

This was the simplest solution I could find until hashicorp/terraform#25609 or hashicorp/terraform#31122 are addressed. Taken from here

Thank you for your time reviewing!

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[bharathkkb]

/gcbrun

[bharathkkb]

Thanks for the PR @loremaps
I think the tobool workaround is a bit confusing. Could we update the documentation instead?

[loremaps]

Hey @bharathkkb thanks for reviewing.

Indeed it is and please feel free to close this PR.

The documentation was updated in #1143 AFAIK to help us avoid this misconfiguration.

But if you miss the docs, please compare the error message shown in #1065 and that of the tobool workaround. And then you have to manually remove from state to recover (see #1065 (comment))

Once again, thanks for your time and feel free to close this, np!

[bharathkkb]

Thanks @loremaps