[META] Support for HTTP/2 (client side)

[reta]

Is your feature request related to a problem? Please describe.
The HTTP/2 support on the server side is getting closer to the first milestone (see please [1]), however the client side is falling behind. The key reason is that all OpenSearch clients (rest-client., RHLC, opensearch-java) use Apache HttpComponents / Core 4.x, which are already EOLed and superseded by Apache HttpClient / Core 5.x. The migration to the Apache HttpClient / Core 5.x is a necessary undertaking (not only to benefit from HTTP/2 support but also to rely on maintanable libraries). In scope of this issue, we need to:

• Migrate to Apache HttpClient / Core 5.x - Migrate client transports to Apache HttpClient / Core 5.x #4256
• Introduce HTTP/2 compatible client transports - [FEATURE] Introduce new OpenSearchTransport based on Apache HttpClient 5 opensearch-java#281

[1] Support for HTTP/2

[dblock]

Would it be easier to remove all REST clients from this repo in 3.0?

[reta]

We could, but we'll keep them (rest, RHLC, ...) anyway, right?

[reta]

@dblock we have only one issue left here [1], please help to get it in, the repo looks unmaintaned

[1] opensearch-project/common-utils#287

[reta]

This is done, closing it