feat: add async overloads of GetManagedObjects()

[jwinarske]

Adds the following to Standardinterfaces.h

template <typename _Function>
PendingAsyncCall GetManagedObjectsAync(_Function&& callback)
{
    std::map<sdbus::ObjectPath, std::map<sdbus::InterfaceName, std::map<PropertyName, sdbus::Variant>>> objectsInterfacesAndProperties;
    return m_proxy.callMethodAsync("GetManagedObjects").onInterface(INTERFACE_NAME).uponReplyInvoke(std::forward<_Function>(callback));
}

std::future<std::map<sdbus::ObjectPath, std::map<sdbus::InterfaceName, std::map<PropertyName, sdbus::Variant>>>> GetManagedObjectsAync(with_future_t) const {
    return m_proxy.callMethodAsync("GetManagedObjects").onInterface(INTERFACE_NAME).getResultAsFuture<std::map<sdbus::ObjectPath, std::map<sdbus::InterfaceName, std::map<PropertyName, sdbus::Variant>>>>();
}

[sangelovic]

@jwinarske Welcome and thanks for the PR. Would be great to also add test cases for the new functionality. Something like SdbusTestObject.GetsZeroManagedObjectsIfHasNoSubPathObjects and SdbusTestObject. GetsManagedObjectsSuccessfully, just that the tests would use the async version. Would you volunteer to write those or shall I help you here?