Make vulnerability filter template extensible and reusable

What does this MR do?

On the security dashboard, there are filters to filter the vulnerability list:

In order to allow more complex filters, this MR splits the filter component into two separate components: a reusable filter_body.vue that handles the rendering of the dropdown and lets the parent component control how it wants to display the dropdown items, and standard_filter.vue, which uses filter_body.vue and keeps the current functionality.

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

Relates to #229661 (closed)