ggcr: use ETags in pkg/v1/remote

[imjasonh]

When pulling a manifest by tag, if the response includes an ETag header, we should hold on to it, and pass it in any future updates to that tag using the If-Match header, as described in this proposal: opencontainers/distribution-spec#250

This prevents race conditions, especially when appending a manifest to an index.

@jonjohnsonjr

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[github-actions]

This issue is stale because it has been open for 90 days with no
activity. It will automatically close after 30 more days of
inactivity. Keep fresh with the 'lifecycle/frozen' label.

[ktarplee]

This would be a really nice feature to add. I think a user of the library could hack this in by modifying the transport but that would be ugly. It would be as simple as adding the ETag (if present) to the Image (or ImageIndex) upon pull. Then extracted (if available) when pushing to a tag and add the If-Match header. Note that this would only apply when pushing to (really updating) a tag (which is the only mutable thing in a registry). It does not make sense for any other type of REST request to the registry.

[imjasonh]

I had an attempt at this here: #1386

It stalled out, but might be worth revisiting if you're interested.