Support passing constant prices in flex context

[Flix6x]

The use case I have for this is to support a control policy to charge immediately, or actually rather, as soon as possible, while still taking into account other constraints such as grid capacity. This is already nicely supported by the storage scheduler, which introduces a small preference to charge sooner rather than later, all else being equal. That is, if prices are constant. Rather than, as a workaround, creating a constant price sensor explicitly, I'd like to convey this in the flex-context, by passing it a constant price.

My idea to support this is to take a step towards harmonizing the flex-context fields with fields in the flex-model that already employ the QuantityOrSensor schema. This specifically means moving from the consumption-price-sensor field to a consumption-price field, which should then support passing any quantity (which can be expressed in some quantity per MWh) or a sensor with any unit. Likewise for production-price.

I have done a tech spike that I'll start a PR with soon. The old fields need to stay alive to avoid introducing a breaking change, but we can adapt our documentation and stop mentioning the old fields.