chore: Enhance contact merge action for identified users (#4886)

- Discard conflicting keys 
- Do not merge if there is already an identified contact

Co-authored-by: Pranav Raj S <pranav@chatwoot.com>

app/javascript/widget/api/contacts.js

@@ -6,8 +6,11 @@ export default {
   get() {
     return API.get(buildUrl('widget/contact'));
   },
-  update(identifier, userObject) {
-    return API.patch(buildUrl('widget/contact'), {
+  update(userObject) {
+    return API.patch(buildUrl('widget/contact'), userObject);
+  },
+  setUser(identifier, userObject) {
+    return API.patch(buildUrl('widget/contact/set_user'), {
       identifier,
       ...userObject,
     });