Define power bound attributes as Power type

### What's needed?

Currently the lower and upper power `Bound` attributes are defined as plain float values. They are used for sending PowerMetrics.

### Proposed solution

Update the`Bound` attributes to type `Power` instead

### Additional context

https://github.com/frequenz-floss/frequenz-sdk-python/blob/v0.x.x/src/frequenz/sdk/timeseries/battery_pool/_result_types.py#L10-L18