🍎 feat: Apple auth (#5473)

* implemented Apple Auth login.

Closes: #3438

TODO:
- write config Doc

* removed some comments

* removed comment

* Add unit tests for Apple login strategy

Introduce comprehensive tests for the Apple login strategy, covering new user creation, existing user updates, and error handling scenarios during the authentication flow. Mocks implemented for external dependencies to ensure isolated testing.

* Remove unnecessary blank line in socialLogins.js

api/models/schema/userSchema.js

@@ -23,6 +23,7 @@ const { SystemRoles } = require('librechat-data-provider');
  * @property {string} [ldapId] - Optional LDAP ID for the user
  * @property {string} [githubId] - Optional GitHub ID for the user
  * @property {string} [discordId] - Optional Discord ID for the user
+ * @property {string} [appleId] - Optional Apple ID for the user
  * @property {Array} [plugins=[]] - List of plugins used by the user
  * @property {Array.<MongoSession>} [refreshToken] - List of sessions with refresh tokens
  * @property {Date} [expiresAt] - Optional expiration date of the file
@@ -111,6 +112,11 @@ const userSchema = mongoose.Schema(
       unique: true,
       sparse: true,
     },
+    appleId: {
+      type: String,
+      unique: true,
+      sparse: true,
+    },
     plugins: {
       type: Array,
       default: [],