HVAC mode in Modbus platform (#23231)

Co-authored-by: Franck Nijhof <frenck@frenck.nl>

source/_integrations/modbus.markdown

@@ -408,6 +408,18 @@ modbus:
         target_temp_register: 2782
         temp_step: 1
         temperature_unit: C
+      - name: "Bedroom Air Condition"
+        address: 10
+        target_temp_register: 10
+        hvac_mode_register:
+          address: 11
+          values:
+            auto: 0
+            cool: 1
+            heat: 2
+            fan_only: 3
+            dry: 4
+        hvac_onoff_register: 11
 ```
 
 {% configuration %}
@@ -454,6 +466,56 @@ climates:
       required: false
       type: string
       default: C
+    hvac_mode_register:
+      description: Definition of a register holding and controlling an HVAC mode
+      required: false
+      type: [map]
+      keys:
+        address: 
+          description: The address of the HVAC mode register.
+          required: true
+          type: integer
+        values:
+          description: A mapping between the register values and HVAC modes
+          required: true
+          type: [map]
+          keys:
+            "off":
+              description: The register value corresponding to HVAC Off mode.
+              required: false
+              type: integer
+            heat:
+              description: The register value corresponding to HVAC Heat mode.
+              required: false
+              type: integer
+            cool:
+              description: The register value corresponding to HVAC Cool mode.
+              required: false
+              type: integer
+            auto:
+              description: The register value corresponding to HVAC Auto mode.
+              required: false
+              type: integer
+            dry:
+              description: The register value corresponding to HVAC Dry mode.
+              required: false
+              type: integer
+            fan_only:
+              description: The register value corresponding to HVAC Fan only mode.
+              required: false
+              type: integer
+            heat_cool:
+              description: The register value corresponding to HVAC Heat/Cool mode.
+              required: false
+              type: integer
+    hvac_onoff_register:
+      description: Address of a register holding and controlling the On/Off state of the climate device.
+        When zero is read from this register, the HVAC state is set to Off, otherwise the `hvac_mode_register`
+        dictates the state of the HVAC. If no such register is defined, it defaults to Auto.
+        When the HVAC mode is set to Off, the value 0 is written to the register, otherwise the
+        value 1 is written.
+      required: false
+      type: integer
     unique_id:
       description: An ID that uniquely identifies this sensor. If two sensors have the same unique ID, Home Assistant will raise an exception.
       required: false
@@ -466,6 +528,12 @@ climates:
 | ------- | ----------- |
 | set_temperature | Set Temperature. Requires `value` to be passed in, which is the desired target temperature. `value` should be in the same type as `data_type` |
 
+### Service `modbus.set_hvac_mode`
+
+| Service | Description |
+| ------- | ----------- |
+| set_hvac_mode | Set HVAC mode. Requires `value` to be passed in, which is the desired mode. `value` should be a valid HVAC mode. A mapping between the desired state and the value to be written to the HVAC mode register must exist. Calling this service will also set the On/Off register to an appropriate value, if such a register is defined. |
+
 ## Configuring platform cover
 
 The `modbus` cover platform allows you to control covers (such as blinds, a roller shutter, or a garage door).