Externalize demo datasets

In order to allow users to create their own demo data sets that can be loaded even after erasing the database and don't require editing Java source code, we need to add possibility to load data from files on file system.

First there will be a property called app.demo.dataset that can be set to a file name. The file will be looked for in optaweb-vehicle-routing-backend/local/datasets. Later we might scan the directory and list all available data sets in the UI.

Additionally, there will be some built-in datasets on classpath and commited to GIT (like the existing Belgium big cities).

We need to come up a with a file format. It should be something hierarchical because the data will probably be heterogeneous (visits, depots, countrycode, osmfile reference?).