[Feature] run build/tests with multiple exist versions

[dizzzz]

In exist-db v5.3 and v5.4 some incompatible API changes have been made, resulting in a failing jfreechart plugin;

As developer I'd like to be able to build (and test) a plugin with multiple versions of eXist so I can detect incompatibilities as early as possible.

[adamretter]

@dizzzz this can be done relatively simply in Maven using Profiles, or you can do this in other more complicated ways via Docker and CI.