Enhancement: Add chart to existing release

As mentioned in: https://github.com/helm/chart-releaser-action/issues/70, in different types of workflows it would be nice to be able to upload to an existing release instead of having to cut a seperate release just for/from CR. Is there any reason that this isn't / shouldn't be the case? Currently when trying to submit to an existing release, I am getting a 422 error.

Would be happy to submit a PR on this, just want to make sure this would be acceptable first.

Start by reviewing the behavior described here and the linked chart-releaser-action issue 70, then trace how uploads to GitHub releases are handled and why an existing release returns HTTP 422. Confirm the intended behavior with maintainers before implementation; done means charts can be uploaded to an existing release without requiring a separate release, with coverage for the relevant workflow.

Written by the indexing model from the issue text.