Correct type for policy stats

[l-trotta]

Originally reported in elastic/elasticsearch-java#1021
policy_stats in GetSnapshotLifecycleStatsResponse.ts is currently mapped as a string array, while actually it's a string of SnapshotPolicyStats objects -> server code.

[github-actions]

Following you can find the validation results for the APIs you have changed.

API	Status	Request	Response
slm.delete_lifecycle	🟢	4/4	4/4
slm.execute_lifecycle	🟢	4/4	4/4
slm.execute_retention	🟢	4/4	4/4
slm.get_lifecycle	🟢	16/16	16/16
slm.get_stats	🟢	4/4	4/4
slm.get_status	🟢	4/4	4/4
slm.put_lifecycle	🟢	4/4	4/4
slm.start	⚪	Missing test	Missing test
slm.stop	🟢	4/4	4/4

You can validate these APIs yourself by using the make validate target.

[github-actions]

Following you can find the validation results for the APIs you have changed.

API	Status	Request	Response
slm.delete_lifecycle	🟢	4/4	4/4
slm.execute_lifecycle	🟢	4/4	4/4
slm.execute_retention	🟢	4/4	4/4
slm.get_lifecycle	🟢	16/16	16/16
slm.get_stats	🟢	4/4	4/4
slm.get_status	🟢	4/4	4/4
slm.put_lifecycle	🟢	4/4	4/4
slm.start	⚪	Missing test	Missing test
slm.stop	🟢	4/4	4/4

You can validate these APIs yourself by using the make validate target.