Plugins: add charger provider

[andig]

This PR allows using chargers as bool getter/setter. This helps the SG Ready use case.

Sample custom SG Ready charger config:

type: sgready
setmode:
  source: switch
  switch:
  - case: 1 # normal
    set:
      source: const
      value: false
      set:
        source: charger
        config:
          type: template
          template: shelly # Shelly 1 für SG-Kontakt
          host: 192.168.178.29
  - case: 2 # boost
    set:
      source: const
      value: true
      set:
        source: charger
        config:
          type: template
          template: shelly # Shelly 1 für SG-Kontakt
          host: 192.168.178.29
  - case: 3 # dimm
    set:
      source: error
      error: ErrNotAvailable
getmode:
  source: calc
  add:
    - source: const
      value: 1
    - source: charger
      config:
        type: template
        template: shelly # Shelly 1 für SG-Kontakt
        host: 192.168.178.29

Temp (soc) and power could also be added to the charger.

/cc @VolkerK62 we would use this to improve the heating docs and finally replace https://docs.evcc.io/docs/faq#heizstab--wärmepumpe

[andig]

@VolkerK62 also added GetMode, basically reads charger.Enabled, converts to int and adds 1. Wdyt of the charger plugin name? I'd rather rename it to switch- does that make sense?