[feat] - use fan with heater

[thurstylark]

My HVAC system needs 3 signals: cool, heat, and fan. I need the fan to be turned on with both the heat and cool modes, but I can't seem to get the fan to turn on when heating.

Here's my config:

  - platform: dual_smart_thermostat
    name: test thermostat
    unique_id: test_thermostat
    heater: input_boolean.test_thermostat_heater
    cooler: input_boolean.test_thermostat_ac
    ac_mode: true
    fan: input_boolean.test_thermostat_fan
    fan_on_with_ac: true
    target_sensor: input_number.test_thermostat_temperature

I tried to get around this by setting up the fan as the secondary_heater as in the following config, but that has the side-effect of causing this component to report "Heating" when it's cooling.

  - platform: dual_smart_thermostat
    name: test thermostat
    unique_id: test_thermostat
    heater: input_boolean.test_thermostat_heater
    secondary_heater: input_boolean.test_thermostat_fan
    secondary_heater_timeout: 00:00:03 
    secondary_heater_dual_mode: true
    cooler: input_boolean.test_thermostat_ac
    ac_mode: true
    fan: input_boolean.test_thermostat_fan
    fan_on_with_ac: true
    target_sensor: input_number.test_thermostat_temperature

Is there a way to enable the same functionality as fan_on_with_ac, but for heating?