http: #15656 #15917 split out and provide glue layers for scala xml and spray-json support

This starts the reorganization of glue classes into fine-grained subprojects as proposed in #15656. In a first step Scala XML support is split out into a new subproject. In the second step, support for spray-json is added in another subproject.

/cc @sirthias 

Should we provide equivalents for `org.w3c.dom.Node`/`javax.xml.SAXSource`? Should be handy for wrapping legacy APIs.