Expose work item custom field values in GraphQL

What does this MR do and why?

Returns the custom fields available to a work item and their corresponding values

References

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• Related to #504530 (closed)

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How to set up and validate locally

1. Enable the custom_fields_feature feature flag

2. Setup some custom fields in http://localhost:3000/groups/flightjs/-/settings/issues

3. There's currently no way to map custom fields to work item types in the UI so you can do it in the console:

   Issuables::CustomField.find(xx).work_item_type_ids = [1] # to enable a custom field on issue types

4. You can also set custom field values from the console since there's no UI for this yet:

   WorkItems::TextFieldValue.create!(work_item_id: xx, custom_field_id: yy, value: 'some text')
   WorkItems::NumberFieldValue.create!(work_item_id: xx, custom_field_id: yy, value: 100)
   WorkItems::SelectFieldValue.create!(work_item_id: xx, custom_field_id: yy, custom_field_select_option_id: zz) # create multiple entries for multi-select

5. Sample query for custom fields widget of a work item

    query {
      namespace(fullPath: "flightjs/flight") {
        workItem(iid: "81") {
          id
          title
          workItemType {
            id
            name
          }
          widgets {
            type
            ... on WorkItemWidgetCustomFields {
              customFieldValues {
                customField {
                  id
                  name
                }
                ... on WorkItemTextFieldValue {
                  value
                }
                ... on WorkItemNumberFieldValue {
                  value
                }
                ... on WorkItemSelectFieldValue {
                  selectedOptions {
                    id
                    value
                  }
                }
              }
            }
          }
        }
      }
    }