[BUG] Failed concurrent creates of ISM policies should return http 409

[phillbaker]

Describe the bug

When creating multiple policies on a fresh AWS OpenSearch cluster, it's likely to hit the following error:

elastic: Error 400 (Bad Request): index [.opendistro-ism-config/4pNn5XhHT3SwvIerntYi-A] already exists [type=resource_already_exists_exception]

Also see phillbaker/terraform-provider-elasticsearch#226

To Reproduce
Steps to reproduce the behavior: This is easy to do with a terraform provider but could also be scripted.

1. Spin up a new cluster
2. create multiple policies in parallel:

curl -X PUT -H 'Content-type: application/json' -d '{"policy":{"default_state":"hot","description":"Rotate index when the size of the index is too big.","ism_template":{"index_patterns":["logs*"],"priority":1000},"states":[{"actions":[{"rollover":{"min_index_age":"7d","min_size":"150gb"}}],"name":"hot","transitions":[{"conditions":{"min_index_age":"14d"},"state_name":"warm"}]},{"actions":[{"retry":{"count":5,"delay":"1h"},"warm_migration":{}}],"name":"warm","transitions":[{"conditions":{"min_index_age":"21d"},"state_name":"delete"}]},{"actions":[{"delete":{}}],"name":"delete"}]}}' localhost:9200/_plugins/_ism/policies/policy_1 & 
curl -X PUT -H 'Content-type: application/json' -d '{"policy":{"default_state":"hot","description":"For logs-rejected index. Smaller size and time to live index","ism_template":{"index_patterns":["logs-rejected*"],"priority":2000},"states":[{"actions":[{"rollover":{"min_index_age":"1d","min_size":"150gb"}}],"name":"hot","transitions":[{"conditions":{"min_index_age":"3d"},"state_name":"delete"}]},{"actions":[{"delete":{}}],"name":"delete"}]}}' localhost:9200/_plugins/_ism/policies/policy_2 & 

Expected behavior
Instead of a http 400, the specific error code http 409 should be returned, indicating a cluster concurrency issue instead of a request format issue.

Plugins
N/a

Screenshots
N/a

Desktop (please complete the following information):
N/A

Additional context
I believe this code could catch a import org.opensearch.ResourceAlreadyExistsException exception and return the specific error code:

index-management/src/main/kotlin/org/opensearch/indexmanagement/indexstatemanagement/transport/action/indexpolicy/TransportIndexPolicyAction.kt

Lines 94 to 101 in 809b3c6

	ismIndices.checkAndUpdateIMConfigIndex(object : ActionListener<AcknowledgedResponse> {
	override fun onResponse(response: AcknowledgedResponse) {
	onCreateMappingsResponse(response)
	}
	override fun onFailure(t: Exception) {
	actionListener.onFailure(ExceptionsHelper.unwrapCause(t) as Exception)
	}

Similar to opensearch-project/security#1402 and opensearch-project/security#1095

[khushbr]

I will start looking into this issue.