Implement `read` and `describe` on the DAE configuration signals

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:

```python
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
}
}
```