v5.16.0

• Don't cancel a mpb.Progress using a context
• Don't recompress non-gzip data without an explicit request
• accept identity tokens from credential helpers
• Add codespell fixes
• Don't initialize a digest.Canonical.Digester if it is not necessary
• Don't compute the (compressed) digest in PutBlob if the caller provides it
• Don't call digest.Digester.Hash() on every digeringReader.Read()
• Pass OriginalDigest and UncompressedDigest to c/storage.Store.PutLayer
• Document that PutBlob callers must only provide validated digests
• Consistently compare docker.Digest with "" without Digest.String()
• Don't unnecessarily compute the blob digest in PutBlob
• copy.Options: semaphore to limit parallel pulls globally