support making HTTP client requests using HTTP/2

See https://github.com/matsluni/aws-spi-akka-http/pull/226#issuecomment-1943538007 for context.

It appears that `Http().singleRequest(...)` only supports sending HTTP/1.1 requests.

We have samples that use HTTP/2 requests but it might be nice to make it more straightforward.
* https://github.com/apache/incubator-pekko-http/blob/main/docs/src/test/scala/docs/http/scaladsl/Http2ClientApp.scala
* https://github.com/apache/incubator-pekko-http/blob/main/docs/src/test/java/docs/http/javadsl/Http2ClientApp.java

Start at Http().singleRequest(...) and compare its current behavior with the Scala and Java Http2ClientApp samples under docs/src/test. Trace how those examples create HTTP/2 requests and determine the integration point for making the client API support them more directly. Done means HTTP/2 requests work through the intended client path and the samples or related checks demonstrate that behavior.

Written by the indexing model from the issue text.