How to iterate over a record batch?

### Describe the usage question you have. Please include as many useful details as possible.

Now I have a `VectorSchemaRoot`. I can see to iterate over the batch with `getVector` and then `getObject`.

But the return value is of type `Obejct`. And I wonder how I can downcast it for some useful class I can retrieve the real value (string, int, float, etc.).

I know we have the field info of each vetcor, but I don't know the mapping between field type to real Java class. It looks over challenge to remember all the mapping by reverse engineering the code, and it may change as version evolves.

I checked https://arrow.apache.org/docs/java/index.html but all the pages tell about constructing a batch and how to move it from one place to another, rather than tell about how to read and dump a batch to a typed two-dimensional matrix.

The most trivial usage, `contentToTSVString`, call `Object::toString` on each cell. But I don't thing we should convert all the values to String and reparse it to concrete type.

Start with the VectorSchemaRoot, getVector, getObject, and contentToTSVString usage described in the issue, then compare them with the Java documentation linked there. Identify the documented way to read each vector with its concrete Java type, and consider the work done when a newcomer can iterate over a batch without reverse-engineering type mappings or reparsing strings.

Written by the indexing model from the issue text.