oneof support with complex types, init of enums

Enums are initialized with 0 and not with null, which caused a data change trigger on some ui elements (furo-ui5-select).

The oneof for the siblings of a complex type are nulled not just initialized. This is done because the grpc gateway expect nothing instead of a empty type.

What's Changed

• FieldnodeAdapter with oneof support by @veith in #89

Full Changelog: https://github.com/eclipse/eclipsefuro-web/compare/@furo/data@2.3.0...@furo/data@2.4.0