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Currently, extensions are initialized in OpenSearch through extensions.yml file. For OpenSearch to process any functionality of the extension be it creating an index, deleting it or any other extension's functionality, the REST APIs of the extension should be registered in OpenSearch (ExtensionsOrchestrator) for OpenSearch to understand which extension it should send request to after receiving it from the user. The RestHandler should do the job of registering those APIs and map it in the sample format mentioned below

{extensionNode1: [API1, API2, API3], extenionNode2: [API1, API4, API5]}

extensionNode will be the node of the extension and values will be the APIs of the extension.

This issue requires us to

1. Register extension REST APIs to ExtensionOrchestrator by building Rest Handler model [3][4]
2. Map the extension node with the APIs it has to register [6] [7]
3. Process the API call sent by the user and send a transport request to the right extension which has the request registered for that API [5] [8]

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