Add ranged readBlob to S3BlobContainer

[ywelsch]

Implements InputStream readBlob(final String blobName, final long position, final int length) on the S3BlobContainer.

[elasticmachine]

Pinging @elastic/es-distributed (:Distributed/Snapshot/Restore)

[tlrx]

LGTM - I left minor comments, most of them can be addressed or ignored as you wish. One is about the name of a blob in a test that needs to be fixed.

[DaveCTurner]

I left a handful of small comments.

I think we should have a test that it is safe to close the stream even if it has not been fully consumed. In earlier experiments we saw some suggestion that the SDK might behave badly if that happens, maybe not recycling the underlying connection properly. If so, we should be sure to consume the whole stream ourselves on close.

[ywelsch]

I think we should have a test that it is safe to close the stream even if it has not been fully consumed. In earlier experiments we saw some suggestion that the SDK might behave badly if that happens, maybe not recycling the underlying connection properly. If so, we should be sure to consume the whole stream ourselves on close.

I've addressed this in ff4361f. It's a larger follow-up change though.

[DaveCTurner]

LGTM except one minor nit, and apparently there's a test failure too.