Conan download_urls endpoints

What does this MR do?

Implements the download_urls endpoints for the conan package registry. These two endpoints return the urls for downloading the conan package files to the conan client, they have the same response as two existing endpoints, so this MR adds the new download_urls endpoints and refactors the duplicate code from the existing endpoints.

Note: This is only a piece of the full conan install implementation, the other install endpoints are in !17798 (merged). These new endpoints sit behind a feature flag, so there is no need for a changelog. The documentation for conan was handled in a separate MR: !16349 (merged)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [ ] Changelog entry
• [ ] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [ ] Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• ~~[ ] Label as security and @ mention @gitlab-com/gl-security/appsec~~
• [ ] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [ ] Security reports checked/validated by a reviewer from the AppSec team

Related #13350 (closed)