API call to get a list of Project's Vulnerabilities

What does this MR do?

This MR adds a Get a list of project's Vulnerabilities REST API call to the newly created Vulnerabilities API. This effort is a part of First-class vulnerabilities backend MVC implementation.

The former Vulnerabilities API is renamed to Vulnerability Findings API (this change will be reflected in docs too but via a separate merge request).

The reasoning behind renaming former Vulnerabilities to Findings and the meaning of newly introduced Vulnerabilities is described in the terminology for the First-class Vulnerabilities which is a huge feature arriving in %12.5.

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