Support secret ref arrays in integration policies

[tobio]

Fixes #1232

[gigerdo]

One thing I noticed with this is that the bug-report also includes a second bug that comes from defining vars_json = "{}",

Because this is defined as nil in the API, there will always be a diff causing the error

│ Error: Provider produced inconsistent result after apply
│
│ When applying changes to elasticstack_fleet_integration_policy.sample_sql_input, provider "provider[\"registry.terraform.io/elastic/elasticstack\"]" produced an unexpected new value:
│ .input[0].vars_json: inconsistent values for sensitive attribute.
│
│ This is a bug in the provider, which should be reported in the provider's own issue tracker.

However this is a separate bug and there is a workaround which is to not define vars_json at all in terraform.