Restore sbt classloader settings

[TimMoore]

#7288 removed some settings from the sbt plugin that allowed for customization of the classloaders used in development mode. These appeared to be unused, but are actually useful for adding dynamic instrumentation of classes via extensions to the Play sbt plugin.

As noted in the Lightbend Telemetry documentation, the instrumentation provided by Cinnamon does not work correctly when running Play apps in development mode because of its reliance on a Java agent. If the classloader hooks are restored, this could be an alternative approach for instrumenting the framework classes. These same hooks could be used by third-parties for similar purposes.

We'll want this capability in Lagom as well. Ideally, it could inherit it from Play.

[TimMoore]

#7773 might be an alternative option