Order Manager Service

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### Problem

The service module `Zuora::Orders` holds service class that generates `orderAction` object for each type ([create a subscription](https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/4310), renewing a subscription, and so on).

The purpose of Order Manager Service is to construct the payload for [Create Orders API](https://www.zuora.com/developer/api-reference/?_ga=2.234436310.991883880.1652218606-397155918.1648069381&_gl=1*9e5pc3*_ga*Mzk3MTU1OTE4LjE2NDgwNjkzODE.*_ga_MY8CQ650DH*MTY1MjI5OTkwOS41MC4xLjE2NTIzMDAyNzYuMA..#operation/POST_Order).

For the `orderActions` object, it can invoke the individual order action services.

### Proposal

<!-- Consult with `@gitlab-com/business-technology/enterprise-apps/zuora-architects` on the ~"Enterprise Applications" team if Zuora business logic is involved. -->

Create `OrderManagerService` service under `Zuora::Orders` module in `app/services`.

Define `#execute` method constructs the payload object with params to create a new subscription (in the scope of this issue):

| Parameter | Type | Description |
|-----------|------|-------------|
| `orderDate` | String | Current date |
| `existingAccountNumber` | String | Account number |
| `subscriptions` | Array of objects | In the scope of this issue, invoke the [service to create a subscription](https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/4310). It will return an `orderActions` object |
| `processingOptions` | Object | Define `run_billing`, `collect_payment`. Refer example below. |

<details>
<summary> Example return object (click to expand) </summary>

```json
          {
            order_date: "2022-05-23",
            existing_account_number: "A00477295",
            subscriptions: [
              {
                "order_actions": [
                  {
                    "type": "CreateSubscription",
                    "trigger_dates": [
                      {
                        "name": "ContractEffective",
                        "trigger_date": "2022-05-23"
                      },
                      {
                        "name": "ServiceActivation",
                        "trigger_date": "2022-05-23"
                      },
                      {
                        "name": "CustomerAcceptance",
                        "trigger_date": "2022-05-23"
                      }
                    ],
                    "create_subscription": {
                      "terms": {
                        "initial_term": {
                          "start_date": "2022-05-23",
                          "period": 12,
                          "period_type": "Month",
                          "term_type": "TERMED"
                        },
                        "renewal_setting": "RENEW_WITH_SPECIFIC_TERM",
                        "renewal_terms": [
                          {
                            "period": 12,
                            "period_type": "Month"
                          }
                        ]
                      },
                      "subscribe_to_rate_plans": [
                        {
                          "product_rate_plan_id": "2c92c0f97700a4f70177060963d923c8"
                        }
                      ]
                    }
                  }
                ]
              }
            ],
            processing_options: {
              "run_billing": false,
              "collect_payment": false
            }
          }
```

</details>

### Result

The service creates the entire payload required to create a subscription using Orders API.

### Next steps (if any)

### How will we measure success?

Story points: 2.0