Add #finding method to Vulnerability

What does this MR do?

In support of #13561 (closed) and !20734 (merged) we need to expose Finding attributes for the Vulnerability object.

This adds the the #finding method on Vulnerability to to make it easier to get the necessary data.

It should also be noted that while we have architected the Vulnerability->Finding relationship as 1-to-many, it will be 1-to-1 for the foreseeable future, which is guiding some of these decisions. For example, in the future, it won't make sense for a Vulnerability to show the details of a Finding (because it may have multiple Findings), but since it is a 1-to-1 relationship for now, there is no where else we would be able to expose that information.

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content