S3A: Support ByteBufferPositionedReadable through vector IO

Make it easy for any stream with vector io to support ByteBufferPositionedReadable

Specifically, ByteBufferPositionedReadable.readFully()

is exactly a single range read so is easy to read.

the simpler read() call which can return less isn't part of the vector API.
Proposed: invoke the readFully() but convert an EOFException to -1