As part of POC of OpenSearch <-> Extension communication we implemented onIndexModule extension point.
But it lacks a lots of features which the extension point is useful for.

Implement onIndexModule[1] extension point for extensions:

• Registration
• Proxy to extension on trigger
• Indices events handling.

[1] https://github.com/opensearch-project/OpenSearch/blob/main/server/src/main/java/org/opensearch/plugins/Plugin.java#L184