Add workflow to sync helm charts to eks-charts

[fawadkhaliq]

Issue
#412

Description of changes
Added a workflow to sync appmesh-controller Helm chart with eks-charts repo.

• Added a script to sync charts if there's a diff. This uses eks-bot to open a pull request on eks-charts repo that will need to be reviewed, approved and merged by the authors. In long term, we plan to deprecate the eks-charts repo and host the charts elsewhere (where? to be designed/decided)
• Added make target to sync on-demand and on release (includes release notes)
• Added GitHub Actions workflow to sync appmesh-controller Helm to eks-chart repo on k8s controller release.

Future work
Since we'll need the same mechanism for other repos (VPC CNI, AWS load balancer controller etc), we will convert the sync script into a GitHub Action that lives by itself and can be shared/imported as a GitHub Action directly in the workflow, removing any code duplication

Testing

• Test syncs from fawadkhaliq/aws-app-mesh-controller-for-k8s to fawadkhaliq/eks-charts repo:
  • https://github.com/fawadkhaliq/eks-charts/pull/1
  • https://github.com/fawadkhaliq/eks-charts/pull/2

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