Add entities for configuration parameters

[dala318]

Once #13 is implemented it will tricky to keep track and setup all input entities and static settings.
Therefor those should be changes to configuration entities created by the integration. To keep track of which entities belong together they should be grouped per instance of planner a "service" (similar as "device")

[dala318]

Some very draft change here https://github.com/dala318/nordpool_planner/blob/config_flow/custom_components/nordpool_planner/number.py