Conan registry check_credentials endpoint

What does this MR do?

This implements the check_credentials endpoint of the Conan registry API, which is an endpoint used by the Conan package manager client. The GitLab implementation has added authentication into the before block of the api, so the credentials are checked before the body of this endpoint, thus the endpoint only needs to return a 200 OK response if it makes it past the before callback.

Note: this MR is the first in a handful of MRs that will eventually implement the entirety of the Conan upload process, the other MRs will be merged into this one if it has not yet been merged into master by the time they are ready to merge. This endpoint is behind a feature flag at the moment.

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 #13345 (closed)