ETag is a string with SHA256 of NeoFS object payload.
Hash is taken from object header by neofs-sdk-go getter.
Checksum type in neofs-sdk-go has changes `String()`
output from `<hash>` to `SHA256:<hash>` in latest update.
S3 gateway should not be relied on unstable string format
implementations and use raw value.
Signed-off-by: Alex Vanin <alexey@nspcc.ru>