[FEATURE] CRUD api for long running operation notification config

[zhichao-aws]

Is your feature request related to a problem?
There are index operations that may take tens of minutes or even hours, e.g. reindex, split. We want to send out notifications to user when operation completed. To track the status of operations, we make some long running operations execute asynchronously and can be tracked by _tasks API(6228), and use IndexOperationListener to watch on .tasks index.

For notification, we think it makes sense that users can customize notification config (i.e. notification channel and message template) on demand for every operation, or just use default notification config.

See more details in https://github.com/opensearch-project/OpenSearch/issues/5479.

What solution would you like?
To support the notification, we can implement CRUD api for long running operation notification config. To customize the notification configs on demand for operations, we can implement CRUD api which binds notification config with task_id. And the dashboard UI can help users manage these configs. For command line users, they can simplify the usage by setting default notification config using CRUD api.

To persist these configs, we need to create a new .admin-panel-config index, which can also be extended for future admin panel features(284).

What alternatives have you considered?
We can also use existing .opendistro-ism-config index by changing its mapping.

Do you have any additional context?
N/A

[zhichao-aws]

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