[C][Client]Support data callback function

[ityuhui]

There is a special API design:
When the client issues a http request, http server will block the request ( does not return to client ), and write some data to stream to client continuously.
e.g.
In kubernetes, when client issues a watch request, kube-apiserver will block this request, but write the watch event data to the stream to client once there is new event such as pod creation/deletion. Client needs to read and handle these event data.

But for now, c generator does not support this mode because apiClient_invoke and writeDataCallback have no chance to access the raw data if the request is blocking.

This PR adds a data_callback_func to writeDataCallback, if it is not NULL, it will be called to access the data received. By this means, user can inject a data_callback_func to handle when the request is blocking.

size_t writeDataCallback(void *buffer, size_t size, size_t nmemb, void *userp) {
    ...
    if (apiClient->data_callback_func) {
        apiClient->data_callback_func(&apiClient->dataReceived, &apiClient->dataReceivedLen);
    }
    ...
}

This change does not impact current request logic because apiClient->data_callback_func = NULL by default.

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@wing328 @zhemant @michelealbano

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[wing328]

CircleCI failure already fixed in the master

[ityuhui]

Thank you @wing328