Supporting assets release types

[MathieuPuech]

It has been introduced in GitLab 13.1, we can now specify the asset type of a link.

See https://docs.gitlab.com/ee/user/project/releases/#links

[nfriend]

@MathieuPuech Agreed, this would be a great feature to have! (And I think it would be relatively easy to implement.)

Would you be interested in opening up a PR for this?

[fgreinacher]

This has been implemented in #242 🎉