feat(socialLink): add support for custom scopes in social account linking (#2074)

* feat(socialLink): add support for custom scopes in social account linking

- Updated documentation to include information on requesting specific scopes when linking social accounts.
- Added a test case to verify that custom scopes are correctly passed to the authorization URL.
- Modified the account linking endpoint to accept additional scopes as an optional parameter.

* chore: lint

* fix(account): update account linking logic and tests

---------

Co-authored-by: Bereket Engida <bekacru@gmail.com>

docs/content/docs/concepts/users-accounts.mdx

@@ -304,6 +304,16 @@ Users already signed in can manually link their account to additional social pro
   });
   ```
 
+  You can also request specific scopes when linking a social account, which can be different from the scopes used during the initial authentication:
+
+  ```ts
+  await authClient.linkSocial({
+      provider: "google",
+      callbackURL: "/callback",
+      scopes: ["https://www.googleapis.com/auth/drive.readonly"] // Request additional scopes
+  });
+  ```
+
   If you want your users to be able to link a social account with a different email address than the user, or if you want to use a provider that does not return email addresses, you will need to enable this in the account linking settings. 
   ```ts title="auth.ts"
   export const auth = betterAuth({