[OAS, later] Callback object support

In OpenAPI 3 specs, you can define callbacks – asynchronous, out-of-band requests that your service will send to some other service in response to certain events.

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 A typical example of a callback is subscription functionality – users subscribe to certain events of your service and receive a notification when this or that event occurs. For example, an e-shop can send a notification to the manager on each purchase. These notifications will be “out-of-band”, that is, they will go through a connection other than the connection through which a visitor works, and they will be asynchronous, as they will be out of the regular request-response flow.

```

The feature is not supported by the initial [C++ Qt5 Client generator:|https://openapi-generator.tech/docs/generators/cpp-qt-client] , but it's a part of OAS 3.*, see:

https://spec.openapis.org/oas/v3.1.0.html#callback-object

We may decide to have that feature implemented for Qt6 version.

See callbacks full example:

https://swagger.io/docs/specification/v3_0/callbacks/