Create a calendar entity for the plan

[JosephAbbey]

Is your feature request related to a problem? Please describe. I have a custom integration called calendar_export which converts a calendar entity to ICS. I would like a predbat plan calendar entity so that it can be converted to ICS and hence displayed in my Google calendar.

Describe the solution you'd like Predbat should create a calendar.predbat_plan entity with the planned charge and discharge periods in it.

Describe alternatives you've considered I thought about making an automation to create the events in a local calendar using the debug plan or the json format from the open pr. But home-assistant does not allow deleting or editing events, so if the plan changes it would break.

Additional context I haven't fully looked into how predbat works with home-assistant entities, but I am sure it must be different from standard integrations so I will understand if this is currently unfeasible.

Thanks! Awesome work!