Add support for a GitHub Bot that watches other repositories and automatically creates charts PRs

[aiyengar2]

Currently, for packages like gke-operator (https://github.com/rancher/charts/tree/dev-v2.6/packages/rancher-gke-operator) that just contain a package.yaml, we need to make a PR to upstream and then make a PR to our charts repository to bump the version.

Instead, we should add support for a GitHub Bot that periodically (daily?) looks at all package.yaml files for autoBumpCharts: true, checks if there is a new Release or GitHub commit to go to, and automatically opens up a PR that bumps the chart and assigns a user to it based on the specified maintainer: GITHUB_USERNAME value in the package.yaml to review and merge (at which point they can also take care of any conflicts that come up).

While this would require some amount of effort to set up (probably would require 2 weeks), this can move us towards a far more automated process for charts.