Add Varbase Auth Base recipe to Varbase Starter

Problem/Motivation

The Varbase Auth Base recipe provides social authentication
and single sign-on (SSO) capabilities using the Social API framework.

It includes integration with providers such as Google, Facebook, LinkedIn.

Since social authentication is a common requirement for modern websites, including it in the Starter template improves developer experience and reduces repetitive setup steps.

Proposed resolution

• #3572229: Initialize Varbase Auth Base recipe
• Add varbase_auth_base to the Varbase Starter site template.
• Ensure required modules (social_api, social_auth, social_auth_google, social_auth_facebook, social_auth_linkedin, etc.) are installed via the recipe.
• Verify compatibility with Drupal CMS Starter and Recipe Installer Kit logic.
• Document how to configure provider credentials after installation.

Remaining tasks

• ✅ File an issue about this project
• ✅ Addition/Change/Update/Fix to this project
• ✅ Testing to ensure no regression
• ✅ Automated unit/functional testing coverage
• ➖ Developer Documentation support on feature change/addition
• ➖ User Guide Documentation support on feature change/addition
• ➖ UX/UI designer responsibilities
• ➖ Accessibility and Readability
• ✅ Code review from 1 Varbase core team member
• ✅ Full testing and approval
• ✅ Credit contributors
• ✅ Review with the product owner
• ✅ Update Release Notes
• ❌ Release varbase-11.0.0-alpha1, varbase_starter-1.0.0-alpha1

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• feat: #3572235 Add Varbase Auth Base recipe to Varbase Starter