feat(dewall_evcharger): add optional Refresh button

This was not listed on issues #3120 and #3152, so does not appear
to be supported on all variants, but it is listed on #4314, and
appears in the log so it looks like we can use its presence as
an availability indicator.

Discussion #4878

custom_components/tuya_local/devices/dewall_evcharger.yaml

@@ -199,3 +199,20 @@ entities:
         type: string
         optional: true
         name: value
+  - entity: button
+    name: Refresh
+    category: config
+    hidden: unavailable
+    dps:
+      - id: 188
+        type: boolean
+        optional: true
+        name: button
+      - id: 188
+        type: boolean
+        optional: true
+        name: available
+        mapping:
+          - dps_val: null
+            value: false
+          - value: true