Charts: Support GitHub Releases

[Proximyst]

Some charts are released on GitHub Releases, e.g. those of some Kubernetes SIGs (example: https://github.com/kubernetes-sigs/security-profiles-operator). They are not on ArtifactHub or any other repository, and as such, aren't possible to reference with today's chart helper.

I believe we should support these, as it is very handy not to have to use a proper repository for some charts, especially where we have no control over it.

The proposed syntax would be one of the following, but can definitely be something entirely else:

# tk tool charts add-repo <name> <url> --github-releases
$ tk tool charts add-repo xyz https://github.com/x/y --github-releases
# (now use it as a normal repository with versions = releases)

# (no repository required)
# tk tool charts add <repository_url> <release>
$ tk tool charts add https://github.com/x/y v1.2.3

# (no repository required)
# tk tool charts add <tgz_url>
$ tk tool charts add https://github.com/x/y/releases/download/v1.2.3/chart-1.2.3.tgz