Use Per Unit Charge Model to create new subscription

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# Bug

## Summary

While purchasing new subscriptions, the price is determined based on the quantity of a service or product purchased.

The current implementation uses [`recurringVolume`](spec/services/zuora/orders/entities/subscribe_to_rate_plans_spec.rb) pricing, while it should use [Per Unit Charge Model](https://knowledgecenter.zuora.com/Billing/Subscriptions/Product_Catalog/B_Charge_Models#Per_Unit_Charge_Model).

Per the [docs](https://knowledgecenter.zuora.com/Billing/Subscriptions/Product_Catalog/B_Charge_Models#Per_Unit_Charge_Model) for Per Unit Charge Model:

> With the per unit charge model, the amount to charge is expressed as a price per unit. The price is calculated based on the quantity of a service or product purchased by the customer, where the total price charged per period would be the quantity multiplied by the per-unit price. An example of a per unit charge would be a software-as-a-service vendor that charges $50 per user per month for their service.

> Per unit pricing can be used with one-time, recurring, or usage charges.

[Docs](https://knowledgecenter.zuora.com/Billing/Subscriptions/Product_Catalog/B_Charge_Models#Volume_Pricing_Charge_Model) for Volume Pricing Charge Model:

> With a volume pricing charge model, the price to be charged is based on the volume purchased. When setting up a volume charge model, a price table is used to define the pricing for each range of volumes, as well as the pricing rule to apply if the customer purchases a quantity that falls within the range of that tier. Each tier is defined by a starting unit, an ending unit, a list price, and a price format (which can be either flat fee or per unit).

Please note that this is **currently not impacting** the existing new subscriptions purchased:

  1. When additional CI minutes is purchased for a new customer (having no prior subscriptions). It uses Order's API workflow for new subscriptions, and the pricing model is `One-Time - Per Unit Pricing`.

![Screen_Shot_2022-08-18_at_9.20.16_AM](/uploads/71a3a5c8377b6a01c68d33403c4e0c8f/Screen_Shot_2022-08-18_at_9.20.16_AM.png)

- [Link to Subscription ](https://test.zuora.com/apps/Subscription.do?method=view&id=8a8aa38682aad91e0182adf139042df4)
- [Verification walkthrough](https://drive.google.com/file/d/1cfBlfMUtXaUK8P4PJg2OayPbz9R_gM3C/view?usp=sharing)

 2. When customer purchases new subscription for a plan. It uses Order's API workflow for new subscriptions, and the pricing model is `Recurring - Per Unit Pricing`.

- [Link to Subscription](https://test.zuora.com/apps/Subscription.do?method=view&id=8a8aa0a082aa6f8c0182ade15fc915f6)
- [Verification walkthrough](https://drive.google.com/file/d/1BpRbusMjgNbMczWDinx9AV8ZboNy0q3i/view?usp=sharing)

This is likely due to the fact that the request also includes [`product_rate_plan_charge_id`](https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/6909e8dfb20722b24f3fbbcf4ea07ba8d5a37a9d/app/services/zuora/orders/entities/subscribe_to_rate_plans.rb#L38) (confirmation pending).

## Steps to reproduce

(How one can reproduce the issue - this is very important)

1. Navigate to the billing page of a namespace in gitlab.com
2. Select a plan and enter pricing details
3. Verify the request defines pricing model as `recurringVolume`

## What is the current *bug* behavior?

Although no impact was observed in the subscriptions created in Zuora, the pricing model in the request needs update.

## What is the expected *correct* behavior?

New subscriptions should be created with `Per Unit Pricing` pricing model.

- for add-ons (CI minutes, storage): use `One-Time - Per Unit Pricing`
- for plans: use `Recurring - Per Unit Pricing`

(What you should see instead)

## Relevant logs and/or screenshots

(Paste any relevant logs - please use code blocks (```) to format console output,
logs, and code as it's tough to read otherwise.)

## Workarounds

(Ways to get around the issue)

## Reported examples

1.
1.
1.

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Story points: 3.0