API call for Create Vulnerability from Finding

What does this MR do?

Implements the Create Vulnerability from Finding API call from the REST API required for backend implementation of the First-Class Vulnerabilities MVC.

This action takes place when a Priority Finding is confirmed and a Vulnerability is created from contents of that Priority Finding.

References:

• the terminology behind Vulnerabilities, Findings, and other entities of the First-Class Vulnerabilities business domain
• the epic

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