New API Support: group level Datadog integration

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New API

API documentation: https://docs.gitlab.com/api/group_integrations/#datadog

Additional Details

• I'd like to contribute it myself

Implementation Guide

The code in client-go has a strong pattern that is repeatable when adding support for new APIs.

• Read the instructions in CONTRIBUTING.md to get your local development environment set up.
• Follow the instructions in AddingAPISupport.md.
  • This file includes instructions for adding all endpoints of an API.
  • It also describes how to write functions for each type of endpoint, so can be used for adding new endpoint support as well.