Fix de-/serialization for AAS with extension(s)

[mjacoby]

This fixes the issue for JSON.
Additional unit tests should be added for XML and AASX.

Superseeds #135

[alexgordtop]

Thanks, that fixes my issue :)

[mjacoby]

• Add java representation of minimal and maximal example (see https://github.com/admin-shell-io/aas-specs/tree/master/schemas/json/examples/generated/Extension) to dataformat-core @mjacoby

  --> impossible to recreate corresponding elements in Java because example JSON is not valid, e.g. using assetKind=notApplicable https://github.com/admin-shell-io/aas-specs/blob/master/schemas/json/examples/generated/Extension/minimal.json#L5

• Update JSON serializer to include new test cases @mjacoby

• Update XML & AASX serializer if required @FrankSchnicke

[mjacoby]

@FrankSchnicke Java classes are now available in dataformat-core (see https://github.com/eclipse-aas4j/aas4j/blob/bugfix/has-extensions/dataformat-core/src/test/java/org/eclipse/digitaltwin/aas4j/v3/dataformat/core/Examples.java). Please add corresponding unit tests for de-/serialization of XML.

[FrankSchnicke]

@mjacoby ready for review