add Await.result optimisation from pekko core

[pjfanning]

See apache/pekko#1666

The main method in pekko-http that can benefit is

pekko-http/http-core/src/main/scala/org/apache/pekko/http/impl/util/package.scala

Line 74 in 6df0cbd

def awaitResult(atMost: Duration): T = {

We don't want to rely on upgrading the pekko core dependency - so we would ideally copy the change to here.

fyi @He-Pin