feat: implement aws-eks-addon datasource

[jzhn]

Changes

Add a new datasource, aws-eks-addon. It uses similar design as existing aws-machine-image and aws-rds to use AWS JavaScript SDK to call APIs to fetch AWS EKS addon versions.

Context

AWS EKS Addons are Kubernetes data-plane components under EKS's management, like kube-proxy and coredns. The addons are usually semver versioned, and released regularly for new features and security patches. AWS EKS provides API DescribeAddonVersions to allow users to query available addon versions per target Kubernetes version.

As platform engineer, I often need to ensure my EKS clusters are running with up-to-date addons. Before this PR, renovate is unable to fetch list of available EKS addon versions that's specific to my cluster version and region. This is a popular ask of renovate users.

Documentation (please check one with an [x])

• [x] I have updated the documentation, or
• [ ] No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• [ ] Code inspection only, or
• [ ] Newly added/modified unit tests, or
• [ ] No unit tests but ran on a real repository, or
• [x] Both unit tests + ran on a real repository
  • 100% unit test coverage
  • tested end-to-end in my self-hosted setup with a terraform project