[BUG] PUT /_plugins/_ism/policies requires a non-standard argument policyID

[dblock]

What is the bug?

In https://github.com/opensearch-project/index-management/blob/395317dcc40762fc05b4320885c2ea27be572324/src/main/kotlin/org/opensearch/indexmanagement/indexstatemanagement/resthandler/RestIndexPolicyAction.kt#L72C33-L72C41 we use policyID instead of what I think should be policy_id.

What is the expected behavior?

Add support for the latter and deprecate the former.

[INFO] => PUT /_plugins/_ism/policies ({
  "policyID": "rollover",
  "if_primary_term": 2,
  "if_seq_no": 3547
}) [application/json] {
...

Do you have any additional context?

Coming from opensearch-project/opensearch-api-specification#578.

[bharath-techie]

We can pickup adding support for policy_id and deprecate policyId based on documentation : https://opensearch.org/docs/latest/im-plugin/ism/policies/