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ARROW-8158: [Java] Getting length of data buffer and base variable width vector

For string data buffer and base variable width vector can we have a way to get length of the data?

For instance, in ArrowColumnVector in StringAccessor we use stringResult.start and stringResult.end, instead we would like to get length of the data through an exposed function.

Now we have getStartOffset in BaseVariableVector and getElementStartIndex/getElementEndIndex in BaseListVector. To be consistent, we add getEndOffset for BaseVariableVector.

Closes #6684 from tianchen92/ARROW-8158

Authored-by: tianchen <niki.lj@alibaba-inc.com>
Signed-off-by: Micah Kornfield <emkornfield@gmail.com>