Implement read and describe on the DAE configuration signals

[rerpha]

If we want sub-values in the XML-encoded DAE configuration signals to be emitted in documents, we need to implement read and describe. We need to do this manually rather than via StandardReadable.

It's not immediately obvious that most of these would ever want to be emitted in documents. But if we do want to do this, an example of how to do this for one signal is:

    async def read(self) -> Dict[str, Reading]:
        reading = await self.period_settings.read()
        period_settings = convert_xml_to_period_settings(reading[self.period_settings.name]["value"])

        ts = reading[self.period_settings.name]["timestamp"]

        return {
            f"{self.name}-periods_soft_num": {
                "value": period_settings.periods_soft_num,
                "timestamp": ts,
            }
        }

    async def describe(self) -> Dict[str, DataKey]:
        return {
            f"{self.name}-periods_soft_num": {
                "dtype": "number",
                "shape": [],
                "source": self.period_settings.source
            }
        }