[Fleet] Provide search bar for fleet agent tags in the dropdown

[Alphayeeeet]

As a Fleet user managing agents, I want to be able to search and sort tags in the dropdown, so that I can quickly find and select the specific tag I need without scrolling through a long, unsorted list.

Context

Currently, the agent tag dropdown displays an unsorted, scrollable list of tags, which can be cumbersome to navigate, especially for users managing many tags. For instance, when a user needs to find a specific tag (e.g., "prod" or "test") from a large list, they currently have to scroll through an unsorted list, which is time-consuming.

Adding a search bar and an optional alphabetical sorting mechanism will significantly improve usability.

To implement this, we can refactor the dropdown UI to use the EUI Selectable component with the searchable option enabled.

Reference: EUI Selectable Component Documentation.

Acceptance Criteria

• [ ] Add a search bar to the agent tag dropdown, allowing users to filter the tag list by typing. The list of tags should filter based on partial match (default EUI Selectable behavior).
• [ ] Refactor the dropdown UI to use the EUI Selectable component with the searchable option enabled.
• [ ] Optionally provide the ability to sort tags alphabetically for easier navigation.
• [ ] Ensure the dropdown supports fast and efficient tag selection for users with a large number of tags.
• [ ] Ensure the updated dropdown meets Elastic's accessibility standards.
• [ ] Retain all existing functionality, including tag selection and the "Clear all" button.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[kpollich]

Great suggestion here and there should be enough prior art to make this happen as a relatively small lift. Going to hijack this to update the description + clarify, then I'll prioritize this.