Add set_heating_curve service

[adorobis]

Introducing a new service to set heating curve of the boiler. Tested and working on Vitodens 200-W WB2B

[adorobis]

Hi @oischinger, do you have any feedback on this feature? Could this be merged?

[oischinger]

Hi @adorobis
Sorry for the late reply. Looks good from my side.
I'll merge the PR and contribute it to HA Core. However I'm not sure if they'll accept it or if they expect slope and shift to be implemented as number entities. Let's see

[adorobis]

Thanks!
Indeed this could be the case but I would find it too easy to change with number entities, even by mistake. Running a service requires more deliberate action. But let's see