Code Review Analytics: Add milestone token for GlFilteredSearch

What does this MR do?

This MR is part of an ongoing effort towards replacing the filtered search bar with GlfFilteredSearch on the Code Review Analytics page, see #212682 (closed) for details.

This MR adds the MilestoneToken component which will be used in the search bar.

This is behind the code_review_analytics_has_new_search feature flag and won't be used yet.

Screenshots

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