Minor Description
For now, just a new 'schema' module that defines common XSD and also an XSD for action invocations.
Eventually (probably not 1.9.0 release) this will evolve into a replacement for the ActionInvocationMemento, as defined by the background service.
To be used by isis addons' isis-module-publishmq for messages (the code in isis-module-publishmq will probably also move into isis core once it's known to be stable).