Added an optinal length field (not handled for now) + Response in case of not supported variant class (exists: false, note: 'Not supported' instead of an error)

Replaced variantClass by alternateBases + Beacon v0.3 as a default + handle all datasets + all datasets as default

First attempt to implement Beacon version 0.3 Updated query parameters (Caution: datasetIds + referenceBase + alternateBase still missing)