onnxruntime

[java] Make the backing byte buffer in an OrtValue accessible (#16578)

### Description
Adds a method to access the backing direct byte buffer from a Java
`OnnxTensor` object, assuming it is backed by a direct byte buffer
(tensors created by ORT's run call or ones created in Java from
multidimensional arrays are not). Also adds a method to check if the
backing byte buffer was copied from the user's buffer supplied on
creation (this could be tested via a pointer comparison from the output
of `getBufferRef` and the user's input buffer, so I'm not sure if it's
necessary).

### Motivation and Context
This is the first part of changes necessary to support output pinning in
Java OrtSession.run/OrtTrainingSession.run calls. I split it out from
the rest of the work as it's useful by itself (e.g. to allow users to
keep a single input tensor and rewrite it each time with new inputs
rather than allocate a fresh one) and the other change will be much more
involved so splitting it makes it easier to review.

cc @yuslepukhin