Adds mock implementation of missing lifecycle mutations

What does this MR do and why?

This MR adds a mock implementation for the following mutations:

1. delete lifecycle mutation
2. attach work item type mutation with status mapping
3. (update mutation) with status mapping

The mock implementation is needed for the 18.4 release to prevent multi-version-compatibility issues during roll out of SM updates.

The implementation performs no action and doesn't have any validation apart from required fields validation and namespace authorization.

References

1. Change lifecycle (#557009 - closed)
2. Delete lifecycle (#557013 - closed)
3. BE: Change lifecycle (#557011 - closed)
4. BE: Delete lifecycle (#557014 - closed)

Screenshots or screen recordings

LifecycleDelete

LifecycleAttachWorkItemType

How to set up and validate locally

LifecycleDelete

Click to expand

mutation ($input: LifecycleDeleteInput!) {
  lifecycleDelete(input: $input) {
    errors
    lifecycle {
      id
      name
    }
  }
}

{
  "input": {
    "namespacePath": "whocka",
    "id": "gid://gitlab/WorkItems::Statuses::Custom::Lifecycle/14"
  }
}

LifecycleAttachWorkItemType

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mutation ($input: LifecycleAttachWorkItemTypeInput!) {
  lifecycleAttachWorkItemType(input: $input) {
    lifecycle {
      id
      name
    }
    errors
  }
}

In the real word using system defined data wouldn't make sense, but it's sufficient for this mock implementation.

{
  "input": {
    "namespacePath": "whocka",
    "workItemTypeId": "gid://gitlab/WorkItems::Type/1",
    "lifecycleId": "gid://gitlab/WorkItems::Statuses::SystemDefined::Lifecycle/1",
    "statusMappings": [
      {
        "oldStatusId": "gid://gitlab/WorkItems::Statuses::SystemDefined::Status/1",
        "newStatusId": "gid://gitlab/WorkItems::Statuses::SystemDefined::Status/2"
      }
    ]
  }
}

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