Get single Vulnerability API call

What does this MR do?

The MR adds the Get single Vulnerability API call to the new Vulnerabilities API that is upcoming in %12.5 as the backstage for MVC Standalone Vulnerability objects.

This API call is covered by the feature flag because it's a part of the massive changes that will be rolled out gradually. The removal of the feature flag is scheduled.

For the same reason, this MR does not provide the documentation for this API call. The documentation is added to the MR that holds the documentation stubs for the new Vulnerabilities API.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry | Changelog will be added when the entire Vulnerabilities API is released
• [-] Documentation created/updated or follow-up review issue created | See !17364 (merged)
• [-] Code review guidelines
• Merge request performance guidelines
• 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

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