feat(releaser): add support for custom krew index repo

[Gearheads]

Summary

This PR is to allow users to provide a custom krew-index repository, in case they do not wish to upload to the public krew-index repository.

• https://github.com/kubernetes-sigs/krew-index

The goal is to allow users to provide the custom krew-index repository information through two new parameters:

• upstream_krew_index_repo_owner
• upstream_krew_index_repo_name

The GitHub Action will then determine the default branch that is configured for the custom krew-index repository, and create a PR against that default branch. Just in case the custom krew-index repository does not use the master branch.

This is very helpful for organizations that are hosting their own internal krew-index repositories, and allows them to automate their release cycles.

Fixes

#47

Please let me know if there is anything else I might be missing. 😄