cluster_policies: Stop allowing Sequence types for value

[Sankalp-Mittal]

Changes

Stop allowing sequence types for values

Why

The backend doesn't do validation at create time for this and hence it becomes important to reject them, as it should not be possible to pass arrays in cluster policies

Tests

unit tests

[github-actions]

Waiting for approval

Could not determine reviewers from git history.
Round-robin suggestion: @lennartkats-db

Eligible reviewers: @andrewnester, @anton-107, @denik, @janniklasrose, @lennartkats-db, @pietern, @shreyas-goenka

_{Suggestions based on git history. See OWNERS for ownership rules.}