Make related issues component more extendable

What does this MR do?

On the standalone vulnerabilities page, we're adding a related issues feature:

that works almost exactly as it does on the Issues page. Rather that re-implement the feature, we are reusing the existing <related-issues-root> component that's used on the Issues page, but the API behavior is slightly different between related issues on the Issues page, and the related issues on the vulnerability page.

This MR is some groundwork to make the <related-issues-root> component more extendable so that it can support both APIs. For reference, this is the follow-up MR that uses the changes in this MR: !35625 (merged)

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

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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done