Extract Vuex module for Dependency Scanning reports

What does this MR do?

This MR extracts the Dependency Scanning report logic in the security_reports store out to a new module. It mirrors closely a previous MR which did the same for SAST.

The newly-extract module is intentionally very similar to the SAST module. This is to ease future work to apply the DRY principle to these modules once they're all extracted.

There are no user-facing changes, it's all ~backstage everything should function in exactly the same way that it did before.

While the MR looks large-ish, it's mostly moved code.

See this issue, or its parent epic for more background.

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