+htp #15930 import RangeDirectives from spray
!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
=pro optimize pull request validation for release-2.3-dev branch

 * enable parallel task execution for everything but tests
 * aggregate only streamAndHttp projects
 * disable publishing
 * don't run extra-build-steps

To be reverted before merging into release-2.3
=str centralize number of runs for randomized stream tests
=str don't run streams test twice because of junit.RunWith annotation
=str #16051 Possibility for Balance to wait for all downstream

* co-authored by @patriknw

balance-fix
Merge pull request #16185 from spray/w/faster-PR-validation

optimize pull request validation for release-2.3-dev branch
Merge pull request #16195 from spray/w/15930-RangeDirectives

+htp #15930 import RangeDirectives from spray
+str #15205 Add FlexiRoute junction
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
Merge pull request #16172 from akka/wip-15205-route-patriknw

+str #15205 Add Route junction
=str #16221 fix graph precondition checks when attaching Source/Sink
Merge pull request #16225 from 2m/wip-16221-graph-source-sink-preconditions

=str #16221 fix graph precondition checks when attaching Source/Sink
=str #16163 allow connecting GraphSource to GraphSink directly

* introduce Identity junction, which is used to connect GraphSource to GraphSink
* only materialize ActorFlowSource and ActorFlowSink as a Flow