Clean up initial data parsing in /buy_minutes route

What does this MR do?

This MR moves the data parsing of /buy_minutes to a util.

Note that this route is behind a feature flag: Feature.enable(:new_route_ci_minutes_purchase) and currently under development. See the Epic here: &5391 (closed).

This MR is dependent on this MR: !59370 (merged) and will be followed by this one: !59371 (merged).

Note to reviewer: Please only review the final commit!

Screenshots (strongly suggested)

With Data (no change)	Without Data (no change)

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 it is 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 #327922 (closed)