Add goreleaser as new material kind

[javirln]

As part of the effort of enabling an automatic way of attesting a GitHub release page, Chainloop internally releases its binaries using goreleaser and attests the internal output. Ideally that output should be added as new material kind.

The goal of the task would be:

• Research the feasibility of adding goreleaser outputs: metadata.json and artifacts.json and decide which one to include if not both of them.
• Check if some kind validations using a json schema could be done.