Determine Ramp Id for a Zuora Subscription

### Background

In [this thread](https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/3968#note_913327995), we're discussing the best way to retrieve Ramp Id for a Subscription using the Zuora REST API.  We mainly need Ramp Id as a way to determine if a Subscription is ramp enabled.

As we've found out, Zuora doesn't make Ramp Id available in many of the Subscription-related API endpoints as mentioned in [the thread](https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/3968#note_913327995).  It's missing from the following endpoints notably:

> * `https://rest.apisandbox.zuora.com/v1/describe/Subscription`: The [v1/describe/\*](https://www.zuora.com/developer/api-reference/#operation/GET_Describe) endpoints are used to determine the schema for each Zuora resource. For Subscription, nothing related to ramps is included in the result.
> * `https://rest.apisandbox.zuora.com/v1/object/subscription/8ad09fc280042bf401801a2c4b1538ec`: The [v1/object/subscription/id](https://www.zuora.com/developer/api-reference/#operation/Object_GETSubscription) endpoint is used by CDot frequently to retrieve data for a particular subscription. It's a key endpoint utilized by the gem/library that CDot uses to interface with Zuora. No ramp information is returned in this API response either.
> * `https://rest.apisandbox.zuora.com/v1/action/query`: The [v1/action/query](https://www.zuora.com/developer/api-reference/#operation/Action_POSTquery) endpoint is used by CDot when filtering is required (e.g. get a list of active subscriptions for a given account id). Constructing a query that references ramp id leads to an error so it seems like Ramp data is not available here either.

We've engaged with Zuora Support to see if they could add this data to these endpoints.  Zuora's response [as noted here](https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/3968#note_941398840) was:

> We are planning to add Ramp ID to this GET Subscription API and are targeting sprint 34, which will be completed by May 20, 2022 and available in production approx 2 weeks after that : https://www.zuora.com/developer/api-reference/#operation/GET_SubscriptionsByKey
>
> Note that it is not the same API that was referenced earlier by Mark (GET Subscription API (https://www.zuora.com/developer/api-reference/#operation/Object_GETSubscription). We are not planning to enhance the Object_GETSubscription API. You can use GET Subscription by Key instead.

[My comment](https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/3968#note_941422221) shown below, speaks to why this might be problematic:

> This is a little disappointing. It's nice that ramp id will be added to https://www.zuora.com/developer/api-reference/#operation/GET_SubscriptionsByKey but that endpoint isn't supported by IronBank (the gem used by CDot). We could write custom code to use this endpoint to get ramp id but at that point it doesn't seem that much different from using https://www.zuora.com/developer/api-reference/#operation/GET_RampsBySubscriptionKey ([mentioned above](https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/3968#note_932690820)) which is already available for use.

In this issue, let's continue the thread [started here](https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/3968#note_913327995) and determine the best path forward to determining if a Subscription is ramp enabled.  I'm breaking this off into its own issue as it will focus the topic and also allow us to close https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/3968 as this issue is [no longer relevant](https://gitlab.com/groups/gitlab-org/-/epics/7099#note_945251257).

### Proposal

Mark Q, Jessica, and I met on 2022-05-24 and agreed on a plan for determining ramp ID for a subscription. For this issue, we will pursue a short-term solution of using the https://www.zuora.com/developer/api-reference/#operation/GET_RampsBySubscriptionKey API endpoint to get the ramp ID for a subscription.  Long-term, this solution will be replaced with using the ramp id from https://www.zuora.com/developer/api-reference/#operation/Object_GETSubscription when Zuora updates this endpoint in late June.  We will open a new issue for the long-term solution.

Story points: 3.0