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

[danielrenninghoff]

Fix #12641

@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.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
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  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
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[wing328]

Local tests compiled without issues. Thanks for the enhancement.