Extend maven repositories snapshot descriptors

Plugin descriptors can define extra maven repositories that must be added to the project pom(s). They are needed to be able to download dependencies for the plugin to work.

Currently the json representation can only process for snapshots "enabled = true/false". You cannot configure the updatePolicy or checksumPolicy (as described on [1]). This is fixed for configured "releases" configuration with the ESSENTIALS-1037.

Fixing this also for snapshots will break backwards compatibility. So that can only be implemented for a release that is part of a new major Hippo release. (Currently the next version will be 12.0.0).

[1] https://maven.apache.org/pom.html#Repositories