REST API: Loadpoint id attribute missing on /api/state

[philipptrenz]

Is your feature request related to a problem? Please describe.
When loading the evcc state via /api/state, the loadpoints are lacking an id property, which is required to, for example, change the mode via POST request.

Describe the solution you'd like
Add id property for each loadpoints array object. I would also suggest to use a unique identifer, like a uuid, for that.

[andig]

For time being, loadpoint id is slice index. Likely to be improved with #12958 in the near future.