Pull Zuora plan ID from CustomersDot API

What does this MR do?

Replaces a hard-coded Zuora plan id with a value retrieved from the CustomersDot application instead.

• Creates a TrialStatusWidgetHelper#ultimate_plan_id method to fetch the Ultimate plan id
• Creates a PaidFeatureCalloutHelper#premium_plan_id method to fetch the Premium plan id
• Uses the GitlabSubscription::FetchSubscriptionPlansService for each of those new methods

Note: The PaidFeatureCalloutHelper is part of an experiment which is currently unreachable (#273627 (comment 674150108)) but is yet to be cleaned up. It is likely that the code for it will be entirely removed. If not, there is already an issue in place to extract the commonalities between these two helpers (#340441 (closed)), including reusing the FetchSubscriptionPlansService data: !70009 (diffs)

How to setup and validate locally (strongly suggested)

1. Enable trials
2. Start a trial
3. On the trial status widget, click on the “Upgrade” button
4. The “Ultimate” plan should be pre-selected

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• [-] 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 #330987 (closed)