provider/circonus: Change check fields from TypeSet to TypeList

[sean-]

[ Migrating this issue from hashicorp/terraform#12808 to this repo. ]

The main reason for this change is the following test failure:

=== RUN   TestAccCirconusCheckCloudWatch_basic
--- FAIL: TestAccCirconusCheckCloudWatch_basic (3.22s)
    testing.go:268: Step 0 error: Check failed: Check 5/191 error: circonus_check.rds_metrics: Attribute 'cloudwatch.1637274235.dimmensions.%' not found

but generally speaking if there's TypeSet which is limited to max. 1 item there's (almost) never a reason to keep it as set as there's nothin to sort - no benefit from the hash.

Such change will require state migration also, because some users may already have existing resources in the state under a hash and we'll need to migrate to the new schema (i.e. TypeList, idx = 0).