Conan file download endpoint

What does this MR do?

This MR implements the endpoints for handling file downloads from the conan package registry. When running a command like conan install, the conan CLI will make api requests to this endpoint.

Note: This is only a piece of the full conan install implementation, the other install endpoints are in !17797 (merged). These new endpoints sit behind a feature flag, so there is no need for a changelog. The documentation for conan is also being handled in a separate MR: !16349 (merged)

This MR is targeting a branch other than master that it builds upon. The other branch is in review right now. Once it is merged, I will re-target this MR to merge into master. Please do not merge before that happens.

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)