Is your feature request related to a problem? Please describe.

OpenSearch currently only supports HTTP/1 which is a single request/response protocol. HTTP/2 allows multiple requests to be in progress at the same time on a single connection with the mechanism of streams. There are workarounds with HTTP/1 such as to maintain multiple open connections or to send fewer large requests (e.g. bulk API). Both has their own problems.

Describe the solution you'd like
HTTP/2 has been around for sometime and is supported in Netty. However, we need to do some work to make sure HTTP/2 works without any issue.

Describe alternatives you've considered
Keep using HTTP/1