[Image Builder] Add download manifest command for airgapped builds

[vignesh-goutham]

Description of changes:
Adds a download manifest command that will pull down all the latest eks-d manifests as well as the eks-a release manifest and all the bundles manifest. Command will create a tarball from all the manifests and finally cleanup.

image-builder download manifest

Last few lines of output

2023/10/09 18:58:01 Downloading eks-a bundles manifest for eks-a version: v0.16.4
2023/10/09 18:58:01 Downloading eks-a bundles manifest for eks-a version: v0.16.5
2023/10/09 18:58:01 Downloading eks-a bundles manifest for eks-a version: v0.17.0
2023/10/09 18:58:01 Downloading eks-a bundles manifest for eks-a version: v0.17.1
2023/10/09 18:58:01 Downloading eks-a bundles manifest for eks-a version: v0.17.2
2023/10/09 18:58:02 Downloading eks-a bundles manifest for eks-a version: v0.17.3
2023/10/09 18:58:02 Creating tarball of downloaded manifests
2023/10/09 18:58:02 Manifest tarball eks-a-manifests.tar was successfully created
2023/10/09 18:58:02 Cleaning up cache build files

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[abhay-krishna]

/lgtm
/approve

[eks-distro-bot]

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This pull-request has been approved by: abhay-krishna

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