System properties provider

The system properties provider is implemented in org.seedstack.coffig.provider.SystemPropertiesProvider. This provider will put all Java system properties as top-level value nodes:

file.separator: "/"
java.home: "..."
java.vendor: "..."
java.vendor.url: "..."

Properties separators (dots) are not used to build a full configuration tree as some nodes would have a value and children at the same time (like java.vendor and java.vendor.url above).

This provider can be used in conjunction with the prefix provider to put all properties under an "umbrella node" like sys.