[C++] Implement ArraySpan support for Variadic Types

[WillAyd]

Describe the enhancement requested

While investigating #44336 I believe I ran into the limitation that the compute modules pass along an ExecSpan object which contains a collection of ArraySpan objects. AFAICT, the ArraySpan is hard-coded to assume 3 fixed buffers, making it difficult to implement compute for variadic types:

arrow/cpp/src/arrow/array/data.h

Line 441 in 8ac434f

BufferSpan buffers[3];

I assume that the ArraySpan therefore needs to be updated to support more than 3 buffers, although if I am going down the wrong path with this happy to be corrected

Component(s)

C++