websocket_client: added example_test with a local websocket server

- Added a example test that connects to a local python websocket server.
- Added readme for websocket_client example.

Closes IDF-907


* Original commit: espressif/esp-idf@67c5225c14

examples/protocols/websocket/main/Kconfig.projbuild

@@ -1,7 +1,21 @@
 menu "Example Configuration"
 
+    choice WEBSOCKET_URI_SOURCE
+        prompt "Websocket URI source"
+        default WEBSOCKET_URI_FROM_STRING
+        help
+            Selects the source of the URI used in the example.
+
+        config WEBSOCKET_URI_FROM_STRING
+            bool "From string"
+
+        config WEBSOCKET_URI_FROM_STDIN
+            bool "From stdin"
+    endchoice
+
     config WEBSOCKET_URI
         string "Websocket endpoint URI"
+        depends on WEBSOCKET_URI_FROM_STRING
         default "ws://echo.websocket.org"
         help
             URL of websocket endpoint this example connects to and sends echo