Namespace storage purchase api

What does this MR do?

Following along with !34746 (merged) this MR is part of #223225 (closed).

You can see in this proposal how and when the API will be used: https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/staging/doc/proposal-storage-limit.md#scenario-5-paid-plan-with-or-without-extra-storage

NOTE: I've not added docs for the API since it will be used by customer portal only. We also don't expose the API for CI minutes yet in our docs.

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

.gitlab/ci/rules.gitlab-ci.yml

@@ -112,9 +112,10 @@
   - "Gemfile{,.lock}"
   - "Rakefile"
   - "config.ru"
-  # List explicitly all the app/ dirs that aren't backend (i.e. all except app/assets).
+  # List explicitly all the app/ dirs that are backend (i.e. all except app/assets).
   - "{,ee/}{app/channels,app/controllers,app/finders,app/graphql,app/helpers,app/mailers,app/models,app/policies,app/presenters,app/serializers,app/services,app/uploaders,app/validators,app/views,app/workers}/**/*"
   - "{,ee/}{bin,cable,config,db,lib}/**/*"
+  - "{,ee/}spec/**/*.rb"
 
 .db-patterns: &db-patterns
   - "{,ee/}{db}/**/*"