Added support for 2nd central heating circuit

[khenderick]

Edit config:

...
external_components:
  source: github://khenderick/esphome-opentherm@feature/ch2
  components: [opentherm]
  refresh: 0s
...
sensor:
  ...
  - platform: opentherm
    ...
    boiler_2_temperature:
      name: "boiler 2 temperature"
    outside_temperature:
      name: "outside temperature"
    oem_error_code:
      name: "OEM error code"
    oem_diagnostic_code:
      name: "OEM diagnostic code"
    ...
  ...
...
binary_sensor:
  ...
  - platform: opentherm
    ...
    ch_2_active:
      name: "CH 2 active"
    service_request:
      name: "service request"
    lockout_reset:
      name: "lockout reset"
    water_pressure_fault:
      name: "water pressure fault"
    gas_flame_fault:
      name: "gas/flame fault"
    air_pressure_fault:
      name: "air pressure fault"
    water_over_temperature_fault:
      name: "water over temperature fault"
    ...
  ...
...
switch:
  ...
  - platform: opentherm
    ...
    ch_2_enabled:
      name: "CH 2 enabled"
    ...
  ...
...
number:
  ...
  - platform: opentherm
    ...
    ch_2_setpoint_temperature:
      name: "CH 2 setpoint temperature"
      min_value: 20.0
      max_value: 45.0
      step: 0.5
      restore_value: true
    ...
  ...
...

Note that after configuring sensors, you need to check the logs for UNKNOWN-messages as typically not all boilers support all messages. Unknown sensors should be removed from the configuration again.