uptake pekko-http trailer header support

* See https://github.com/apache/pekko-http/pull/684
* pekko-grpc has similar code
* May be better to not hurry to do this. Having a bit of code duplication may be better tightly binding pekko-http and pekko-grpc - unless the pekko-http code is much better than the pekko-grpc equivalent

Start by reviewing pekko-http PR 684 and the similar code in pekko-grpc. Compare the two approaches and clarify whether trailer header support should be implemented in pekko-http or kept duplicated; done means the chosen approach is agreed and pekko-http supports trailer headers accordingly.

Written by the indexing model from the issue text.