Add support for ARM64

[hakman]

1. Describe IN DETAIL the feature/behavior/change you would like to see.

AWS is ready to GA the Graviton 2 instance types (C6g, M6g, R6g).
Performance looks great and may become quite popular, so it would be nice if Kops could use them as worker nodes.

Benchmarks:
https://www.anandtech.com/show/15578/cloud-clash-amazon-graviton2-arm-against-intel-and-amd/10

2. Feel free to provide a design supporting your feature request.

Kops generates a list of assets that is added to the Additional User Data together as part of the NodeUp config. At the moment this only provides AMD64 assets, but could provide also other architectures like ARM64.
NodeUp should easily detect the OS architecture and pull the corresponding assets.
As long as there is a Docker version that supports ARM64, the node should start and join the cluster without any issues.

[hakman]

/assign