Generate vulnerability ID

What does this MR do?

Customize JSON marshaling in order to add an id field to vulnerabilities. The id is the SHA-256 hash of a string that combines all the fields of the Issue struct, and that excludes id since it's not a struct field. Ideally the id should be a randomly generated UUID but right now it has to be predictable because of the current implementation of klar and gemnasium - see dedicated discussion.

• Bump minor of format version
• Add new Issue.ID() function
• Add new id field to JSON vulnerabilities, dynamically set to Issue.ID()
• Add Ref.ID, to be serialized as id
• Provide helper function NewRef to easily create references
• Say about CompareKey being deprecated, in the code comments

What are the relevant issue numbers?

gitlab-org/gitlab#36777 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• [-] Documentation created/updated for GitLab EE, if necessary
• [-] Documentation created/updated for this project, if necessary
• [-] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug; See integration tests
• [-] Job definition updated, if necessary
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer