[Follow-up][BUG] Extension requests show false timeouts on first few requests

[DarshitChanpura]

This is a follow-up for #8652 to ideate and implement the solution.

During research of #8652 we discovered that the even though the extension send the response to OpenSearch, OS would still return No response.. message to the requester. Upon further analysis it was discovered that inProgressFuture.orTimeout(ExtensionsManager.EXTENSION_REQUEST_WAIT_TIMEOUT, TimeUnit.SECONDS).join(); would always timeout on trying to join() back to main threadPool. See more about this in the flow diagram in this comment: #8652 (comment)

Upon conclusion of research with @reta, it was discovered that the way RestSendToExtensionAction#prepareRequest is implemented would need some changes. These changes are proposed here: #8652 (comment)
This solution might not be a straightforward fix since Extension expects requests from OpenSearch at Transport Layer, but this issue should be used to ideate and develop the fix.

Additional context

Steps to reproduce:

• Checkout main and start helloWorld extension from main on SDK
• Start OpenSearch
• Initialize the extension following DEVELOPER_GUIDE
• Hitting curl -XGET http://localhost:9200/_extensions/_hello-world/hello should produce No response from extension to request.%
• You should also see this in OpenSearch logs:

[2023-07-11T19:11:12,938][INFO ][o.o.e.r.RestSendToExtensionAction] [runTask-0] Forwarding the request GET /hello to {hello-world}{hello-world}{UB8MdMJMRoGqXkceAEFC_g}{127.0.0.1}{127.0.0.1:4532}{dimrs}
[2023-07-11T19:21:22,969][INFO ][o.o.e.r.RestSendToExtensionAction] [runTask-0] Received response from extension: OK

• After first 5-6 requests that "false" timeout after 10s, all the requests that follow should process as normal.

Logs

• OS server and Extension logs: [Research] [BUG] Extension requests show "false" timeout for first few requests to extension due to unknown bug #8652 (comment)
• Debug server output: [Research] [BUG] Extension requests show "false" timeout for first few requests to extension due to unknown bug #8652 (comment)