Add item selector namespace dropdown

What does this MR do and why?

Adds a dropdown to select between Project and Groups in the Item Selector component

Screenshots or screen recordings

No visual changes are expected from this MR as the component is not used yet.

This is what we're working towards:

Before	After

How to set up and validate locally

No visual changes are expected from this MR as the component is not used yet.

If you still want to test you can just add the component to any Vue app: <list-selector title="test" type="users" :selected-items="[]" :project-path="projectPath" />

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #428519 (closed)