[C++][Qt]Replace usage of QVariant with a more intuitive optional wrapper

Created by: etherealjoy

Fixes #8934 (closed)

Issue was due to Optional support with QVariant here #8733

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Changes

• QVariant cannot construct user types hence it is not suitable for all cases
• A more intuitive approach is provided, which also shows the type of optional in the api instead of plain QVariant
• Added handling of optional body

Function declaration now looks like this

void deletePet(const qint64 &pet_id, const OptionalParam<QString> &api_key = OptionalParam<QString>());
void updatePetWithForm(const qint64 &pet_id, const OptionalParam<QString> &name = OptionalParam<QString>(), 
           const OptionalParam<QString> &status = OptionalParam<QString>());

instead of this

void deletePet(const qint64 &pet_id, const QVariant &api_key = QVariant());
void updatePetWithForm(const qint64 &pet_id, const QVariant &name = QVariant(), const QVariant &status = QVariant());

User types are supported like this

    QList<OAIObject> list;
    QMap<QString, OAIObject> map;
    QList<QString> strl;
    list.append(OAIObject());
    api->notifyTopicV1(list);
    api->notifyTopicV1();
    api->notifyTopicV2();
    api->notifyTopicV2(map);
    api->notifyTopicV3();
    api->notifyTopicV3(strl);

cc @ravinikam @stkrwork @MartinDelille @muttleyxd

@xconverge I had to rework this due to side effects, made a more simpler and intuitive approach.