kuberentes mixin should be able to reference a URL

[carolynvs]

The kuberentes mixin should be able to reference a manifest from a canonical source, like a URL. Then during porter build, the mixin should handle copying the manifest from that source to the invocation image so that it is in the bundle and then at runtime, the mixin uses the cached file instead of the URL.

We don't yet have a solution for how to reference the file at runtime. See getporter/porter#218 and getporter/porter#621 for context which both are intended to provide generic solutions for mixins to these problems.

[squillace]

Helm repos, too. I'll modify the https://github.com/squillace/porter-helm3 with the same functionality....