Add ble RSSI sensor for connected devices (#3860)

tests/test1.yaml

@@ -321,6 +321,7 @@ mcp23s17:
 
 sensor:
   - platform: ble_client
+    type: characteristic
     ble_client_id: ble_foo
     name: Green iTag btn
     service_uuid: ffe0
@@ -335,6 +336,11 @@ sensor:
       then:
         - lambda: |-
             ESP_LOGD("green_btn", "Button was pressed, val%f", x);
+  - platform: ble_client
+    type: rssi
+    ble_client_id: ble_foo
+    name: Green iTag RSSI
+    update_interval: 15s
   - platform: adc
     pin: A0
     name: Living Room Brightness