Add Vuex store for milestone combobox

What does this MR do?

To help make upcoming changes to the milestone combobox (#232569 (closed) & #232577 (closed)) easier, this MR adds a Vuex store for the milestone combobox component to use.

These changes were originally in !43427 (merged), but the MR grew to very large, so I split adding the Vuex store into this MR, while !43427 (merged) will be for migrating to the Vuex store.

This MR doesn't change the UI.

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