Moebot S: add buttons for commands inaccessible through lawn_mower.

Lawn Mower platform is implemented in a difficult to extend manner.
Add Fixed mowing and Cancel buttons to access the additional commands.

Document lawn_mower entity.

Issue #1068

custom_components/tuya_local/devices/README.md

@@ -590,6 +590,11 @@ Humidifer can also cover dehumidifiers (use class to specify which).
 - **humidity** (optional, number): a dp to control the target humidity of the device
 - **current_humidity** (optional, number): a dp to report the current humidity measured by the device
 
+### `lawn_mower`
+- **activity** (required, string): a dp to report the current activity of the mower.  Valid activities are `mowing`, `paused`, `docked`, `error` (from LawnMowerActivities in https://github.com/home-assistant/core/blob/dev/homeassistant/components/lawn_mower/const.py).  Any additional activities should be mapped to one of those, and exposed through an extra attribute or sensor entity that shows all the statuses that the mower is reporting.
+
+- **command** (required, string): a dp to send commands to the mower.  Recognised commands are `start_mowing`, `pause` and `dock`.  Any additional commands should be implemented via a `button` or `select` entity.
+
 ### `light`
 - **switch** (optional, boolean): a dp to control the on/off state of the light
 - **brightness** (optional, number 0-255): a dp to control the dimmer if available.

custom_components/tuya_local/devices/moebot_s_mower.yaml

@@ -122,6 +122,28 @@ secondary_entities:
             value: stop
           - dps_val: StartReturnStation
             value: return_to_base
+  - entity: button
+    name: Start fixed mowing
+    icon: "mdi:mower-on"
+    dps: 
+      - id: 115
+        type: boolean
+        name: button
+        optional: true
+        mapping:
+          - dps_val: StartFixedMowing
+            value: true
+  - entity: button
+    name: Cancel mowing
+    icon: "mdi:mower"
+    dps: 
+      - id: 115
+        type: boolean
+        name: button
+        optional: true
+        mapping:
+          - dps_val: CancelWork
+            value: true
   - entity: sensor
     class: battery
     dps: