!htp #15656 split out scala-xml support into its own subproject
+pro #15917 provide SprayJsonSupport in akka-http-spray-json project

This will serve as an example of how to provide similar glue layers for
other JSON libraries.
+htc #16084 introduce CustomHeader superclass for user-defined headers
Merge pull request #16205 from spray/w/15656-fine-grained-dependencies

http: #15656 #15917 split out and provide glue layers for scala xml and spray-json support
Merge pull request #16207 from spray/w/16084-CustomHeader-model

+htc #16084 introduce CustomHeader superclass for user-defined headers
=str  #16109: Replace ScalaGraph with custom graph impl
Merge pull request #16193 from drewhk/wip-16109-new-graph-lib-drewhk

=str  #16109: Replace ScalaGraph with custom graph impl
=str #16221 loosen graph precondition checks when attaching Source/Sink