This change adds a new manifest-v2/ path for new models created with the
create/pull/copy commands. Under manifest-v2, manifests are now just blobs which are
content addressable similar to tensors/config files. The named tags instead
will symlink/hard link/contain a copy depending on what the file system supports.
Downgrades to older versions of ollama are still possible, but any create/pull/copy
done with the newer version will potentially have its blobs pruned by the older
version.
manifest-v2 also changes the default registry name to `ollama.com` instead of
`registry.ollama.ai`.
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