Adds multiarch builds for k8s http2 example

[BennettJames]

This modifies the images used for the k8s http2 example to default support arm64 in addition to amd64. This should gracefully downgrade to existing behavior if run from an environment that does not support cross-building.

This is done on this example as it's the "main" k8s setup for walkthroughs for demoing multiarch k8s abilities. Further walkthroughs may be adapted in the future.

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Description of changes:

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