ggcr: use ETags in pkg/v1/remote

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: https://github.com/opencontainers/distribution-spec/issues/250

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

@jonjohnsonjr

Start in pkg/v1/remote and trace manifest-by-tag reads and later tag updates. Verify that an ETag from a pull is retained and sent as If-Match on future updates, covering the race-prevention behavior described in the linked distribution-spec proposal.

Written by the indexing model from the issue text.