[FEATURE] Support _cat/allocation API on per cluster metrics monitor

[spapadop]

Is your feature request related to a problem? We are a big fun of the "per cluster metrics monitor" type. I've seen the list of supported APIs growing, which is very nice. One API that we would particularly enjoy to have support for is the GET _cat/allocation one, which is not supported for now.

What solution would you like? Add GET _cat/allocation API on "per cluster metrics monitor" type.

[praveensameneni]

Adding to backlog to support _cat/allocation in https://github.com/opensearch-project/alerting/blob/d7ee9717366a1c224cbf8f6611064ea9edbe3455/alerting/src/main/resources/org/opensearch/alerting/settings/supported_json_payloads.json#L4