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GH-37199: [C++] Expose a span converter for Buffer and ArraySpan (#38027)

### Rationale for this change

Convenience. We can have such a helper at the buffer and array data level.

### What changes are included in this PR?

Add `Buffer::span_as`, `Buffer::mutuable_span_as`  and `ArraySpan::GetSpan`.

### Are these changes tested?

No,  but I'm happy to add some test if needed.

### Are there any user-facing changes?

Yes, new public functions.
* Closes: #37199

Authored-by: jsjtxietian <jsjtxietian@outlook.com>
Signed-off-by: Felipe Oliveira Carvalho <felipekde@gmail.com>