[QtNetwork] std::chrono support for transferTimeout values

The QNetworkAccessManager and QNetworkRequest classes have setter-getters
for 'transfer timeout' value. They use 'int' of milliseconds as the interface type.

This task is about adding support for std::chrono::milliseconds support as a datatype.
This means overloading the setTransferTimeout() setter, and introducing a new getter function as we can't overload based on the return type.

Notably, as an aside, the QTimer doesn't currently support timeouts that are larger than 'int' of milliseconds. This means that the behavioural limitation remains (maximum timeout on an int32bit system is ~24 days).
But nevertheless, adding std::chrono as an interface type provides users with more modern means of expressing time.