[cpp-qt-client] Add support for AnyType objects

Created by: danielrenninghoff

Fix #12641 (closed)

@ravinikam @stkrwork @etherealjoy @MartinDelille @muttleyxd

The cpp-qt-client generator generates invalid code when the OpenAPI document contains AnyType objects (example: type: {}). I have replaced AnyType with QJsonValue, so that the correct type can be manually unpacked later.

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