Add Buildmetadata as Metadata

Buildmetadata created with the Buildmetadata Maven Plugin should be made available as metadata. Currently we do not need to have a human readable report, just the metadata to reference.

Each buildmetadata property name is prefixed by "buildmetadata.". It is the linearisation of the XML document that is stored inside the "JAR/META-INF/buildmetadata.xml". The properties file is not used (since the other keys are generated).