Global Search - Searchable Dropdown

What does this MR do?

This is work broken off a larger MR here: !45711 (merged)

When implementing the above linked MR I found a significant case of duplicated and overly complex code. This change creates a generic component SearchableDropdown that will be used by the Group and Project filter on Global Search.

The Group filter was previously implemented here: !44833 (merged)

This will allow us to not only remove a large set of duplicated code, but also allow us to keep these filters aligned in style to avoid issues like: #285229

Screenshots (strongly suggested)

Note no visible change should be found

	Before	After
Any
Selected Group
Searched Grouip

GIF

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team