Suntec Coolfixx: add temperature unit select entity

- also make temp_current_f optional, as it seems to be missing when
unit is set to c.

Issue #1071

custom_components/tuya_local/devices/suntec_coolfix_airconditioner.yaml

@@ -93,6 +93,7 @@ primary_entity:
     - id: 24
       type: integer
       name: temp_current_f
+      optional: true
     - id: 101
       type: boolean
       name: preset_mode
@@ -136,4 +137,18 @@ secondary_entities:
           - dps_val: 4
             value: true
           - value: false
+  - entity: select
+    name: Temperature unit
+    icon: "mdi:temperature-celsius"
+    category: config
+    dps:
+      - id: 19
+        type: string
+        name: option
+        mapping:
+          - dps_val: c
+            value: Celsius
+          - dps_val: f
+            value: Fahrenheit
+