Feat(subscriptions helper): fetch ci minutes data

What does this MR do?

This MR introduces a new service that exposes Plan data from CustomerDot (see implementation here) and uses it to send CI Minutes Plan data to the subscriptions pages.

Screenshots (strongly suggested)

This is purely functional, but you enable this feature flag: Feature.enable(:new_route_ci_minutes_purchase), apply this patch: log-ci-minutes-data-in-buy-minutes-route.patch and navigate to http://localhost:3000/-/subscriptions/buy_minutes. Open the console and you should see the data be printed.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this route is still behind a feature flag.
• [-] 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

Related to #323357 (closed)