Implement DirectAssetPath for Release Assets Links

[timofurrer]

This change set adds support for the direct_asset_path API field for Release Assets Links, see the API documentation here:

• https://docs.gitlab.com/ee/api/releases/#create-a-release
• https://docs.gitlab.com/ee/api/releases/links.html#create-a-release-link

[timofurrer]

@svanharmelen do you mind giving this a review and creating a release? We require this in order to unblock 17.0 deprecations. Thanks a bunch!