Retain executable flag on files in bundle

[benmoss]

Describe the problem/challenge you have
Right now when you do an imgpkg push to create a bundle it will drop the executable flag on any files, it is hard-coded to create every file in the tarball with 0600: https://github.com/vmware-tanzu/carvel-imgpkg/blob/499b334d53c78a9624f427e96dcb6e1a87dd6d50/pkg/imgpkg/image/tar_image.go#L129

Describe the solution you'd like
The executable flag should be kept on the files

Anything else you would like to add:
Slack thread: https://kubernetes.slack.com/archives/CH8KCCKA5/p1614181788053900

[joaopapereira]

Looks good to me.

We should ensure the following:

• Only add execution permission to files that had execution permission previously

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[aaronshurley]

When you pick up this story, discuss the possibility of keeping the ownership bits.

[joaopapereira]

Shipped with version 0.7.0