Planner: add a meta plan handler

[andig]

Fix https://github.com/evcc-io/evcc/issues/24497

The meta plan handler provides sequential handling for multiple concurrent plans in a load management-restricted environment. It does so by calculating the total plan duration for all active plans and creating an optional overall plan. The overall plan is then sliced into individual plans.

TODO

• [x] calculate meta plan
• [ ] improve calculating meta plan by step-wise combination of overlapping plans
• [ ] visualize meta plan
• [ ] Make loadpoint planner use meta plan slots

[iseeberg79]

Is it also relevant to take the load-management circuit of loadpoints into account and group those?

[RenatusRo]

Maybe I'm wrong, but the SmartCost limit periods are implicit plans. Currently, plans can overrun when (under load restrictions) smart charging kicks in on other LPs.