Allow anonymous access to public Conan packages

What does this MR do?

For Conan API endpoints:

• Allow access to ping endpoint without a token
• Allow access to download endpoints without a token, if the project is public
• Instead of throwing UnauthorizedError when no token is provided, continue with anonymous privileges
• Update tests to reflect the above changes
• Add test for downloading without a token

See #294482 (closed) for details.

Screenshots

(no visual changes)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• [ ] Merge request performance guidelines — no expected performance changes
• Style guides
• [ ] Database guides
• [ ] Separation of EE specific content

Availability 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
• [ ] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

• 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 to #294482 (closed)