IQMBackend improvements

[hay-k]

• Adds a new option called circuit_callback to IQMBackend. Description and motivation is in the user guide.
• Unknown options passed to backend now cause an error. Previously they were just silently ignored, which was not good. E.g. if you wanted to specify custom calset ID, but you made a typo in the name of the option, it would get ignored and the default calibration set would be still used. And you would be left wondering why your calibration set is not being used. Now you will immediately get error that you have not supplied a correct option.
• Handling of options is now improved a bit and the code is easier to read and consume.
• Fix the type of date in result object. Qiskit expects it to be string. It was datetime before.
• IQMBackend can now serialize x, rx, y and ry gates as well. More details and motivation in the docstring of the serialize function.
• Improved the layout of user guide.