Code Review Analytics: Add Vuex store to app

What does this MR do?

In !22534 (merged) we added the search bar on the Code Review Analytics page, see #38062 (closed). This MR adds the Vuex store and updates the store when the user filters by milestone or label.

The MR table will be added in a separate MR.

This is behind the code_review_analytics feature flag.

Screenshots

N/A

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

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