Turn market_id attribute into consumption_price_sensor

[nhoening]

          I was [planning](https://github.com/FlexMeasures/flexmeasures/discussions/763#discussioncomment-6500979) to document which attributes are actually relevant, and market_id would have to be added to the list.

Is it possible to replace the name market_id with consumption-price-sensor, for instance, so we get more consistency?

Originally posted by @nhoening in #695 (comment)

As @Flix6x said, this needs a db migration, a few code replacements and maybe a check within plugins we have, like the one we use for simulations.

Plus, maybe mentioning in the upgraded documentation that consumption-price-sensor can also live outside of the flex context as an attribute.
Or (my preferred option), we use flex-context["consumption-price-sensor"], for more consistency. I want us to look up flex-model and flex-context on either sensor or asset, and this way we can get rid of market_id with a comprehensive solution which will work for all other things, as well (capacity_in_mw would be part of flex-model, see #335 - we could tackle these together)

[nhoening]

@Flix6x suggests doing this together with other renaming work, see #911