Add "kops get assets" command

[johngmyers]

This is rough around the edges, but gives the idea.

[hakman]

Small suggestion, maybe skip the mirror field when not needed.
Also, would be nice to move this to klog.V(1).Info():

I0529 08:59:26.979910   39040 dns.go:97] Private DNS: skipping DNS validation

[hakman]

Suggested change

	Mirror: containerAsset.DockerImage,
	}
	}
	if containerAsset.DockerImage != containerAsset.CanonicalLocation {
	image.Mirror = containerAsset.DockerImage
	}

[hakman]

Suggested change

	Mirror: fileAsset.DownloadURL.String(),
	SHA: fileAsset.SHAValue,
	}
	SHA: fileAsset.SHAValue,
	}
	if fileAsset.DownloadURL.String() != fileAsset.CanonicalURL.String() {
	file.Mirror = fileAsset.DownloadURL.String()
	}

[hakman]

Suggested change

	Mirror string `json:"mirror"`
	Mirror string `json:"mirror,omitempty"`

[hakman]

Suggested change

	Mirror string `json:"mirror"`
	Mirror string `json:"mirror,omitempty"`

[johngmyers]

I support suppressing chatter like that DNS log line, though I think that's perhaps a different PR.

On the mirror field, I'm a little wary of requiring consumers of that data to implement special cases for edge cases like this.
Perhaps I'm using the wrong terminology? Should I stick with "Download" and "Canonical"?

I could see a desire for less information when downloading from the canonical location, but then I don't see any use cases for this command in that situation.

[hakman]

I understood perfectly the terminology, no worries. Though the name of the field "mirror" is misleading. This reminds me of the mirrored assets we already have.
One can use this to inspect what assets will be downloaded in advance, for example to see what CNI or addon versions will be used by default. Another use case (more popular these days) is for building AMIs with these assets pre-loaded.

[johngmyers]

@hakman the concern wasn't about understanding, but with which terminology was appropriate for the feature. I've changed it to be more consistent and accurate about what the values are.

[hakman]

Agreed. Thanks!
Feel free to cherry-pick to any release.

[k8s-ci-robot]

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[johngmyers]

It's a bit late to add features to 1.21.