Application Passwords on Migrations: Create success callback endpoint

[valterlorran]

More context: paYKcK-5B6-p2

In the migration flow, when users request authorization for the Application Password, they are redirected back to WordPress.com after confirming their request. In this case, we need an endpoint that will trigger the chain of jobs for the assisted flow. As the main one, create the admin user with the Application Passwords. Please remember that we'll not store the Application Passwords, and we should also queue the job to remove them once we successfully queue the chain of the Assisted Migration jobs.

We may want to protect the callback URL using nonces: https://developer.wordpress.org/apis/security/nonces/. Hence, making it necessary to generate the callback URL in the back end as I suggested here: p1731981573137839-slack-C0Q664T29

[github-actions[bot]]

OpenAI suggested the following labels for this issue:

• [Feature Group] Site Migration: The issue is focused on creating a callback endpoint within the migration flow for Application Passwords, which is directly related to site migration functionalities.
• [Feature] Authentication: The issue discusses the process of requesting and using Application Passwords, which falls under authentication mechanisms.
• [Feature] User Management: Creating an admin user and managing Application Passwords relates to how users are handled within the system.
• [Feature] Site Settings: The implementation involves specific settings regarding the management of Application Passwords during the migration process.