View History of All Project Imports - Backend"

What does this MR do and why?

Related to #331567 (closed)

This MR adds import_url field and imported filter to our REST API projects endpoint This is required to implement project history page where we need to list all imported projects history

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Import project from anywhere (git url, github, etc. ok)
2. Ensure it is present in /api/v4/projects?imported=true
3. Ensure import_url field is included

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.