Add submodule reference to `SD-DSS`

The `SD-DSS` is not yet available as a maven package, so we use a git submodule to load it
and a module in the main `pom.xml` to reference and build it.
The submodule reference will be replaced with a maven one as soon as it becomes available
(see https://joinup.ec.europa.eu/software/sd-dss/topic/how-contribute#comment-15030)