IAV-280/feat(collaborator): Add collaborator-agent linking functionality and cleanup unused stripeCustomer code

Summary

This MR introduces the ability to link and unlink collaborators to agents, and removes obsolete stripeCustomer references from the Admin module.

Changes

New Features

• Added PUT /collaborator/{uid}/link-to-agent endpoint to associate a collaborator with an agent
• Added PUT /collaborator/{uid}/unlink-from-agent endpoint to remove the association

Code Cleanup

• Removed stripeCustomer-related code from AdminService and AdminRepository as it is no longer part of the Admin entity

Testing

• Link collaborator to agent
• Unlink collaborator from agent

Related Issue

Closes IAV-280