Add loading prop to filter body and fix search box clear icon

What does this MR do?

On the vulnerability report, we have some dropdowns that filter the vulnerability list:

These dropdowns use a <filter-body> component that internally uses <gl-dropdown>. This MR improves the component by doing the following:

1. Add a loading prop and a <GlLoadingIcon>:

2. Fix a bug where the search box was not showing the clear icon when there is text in the search box:

Before	After

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • [] I have included a changelog entry.
  • I have not included a changelog entry because the changes are minor and this is mostly setup work for a future MR.
• 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