arrow

GH-35498: [C++] Relax EnsureAlignment check in Acero from requiring 64-byte aligned buffers to requiring value-aligned buffers (#35565)

### Rationale for this change

Various compute kernels and Acero internals rely on type punning.  This is only safe when the buffer has appropriate alignment (e.g. casting uint8_t* to uint32_t* is only safe if the buffer has 4-byte alignment).  To avoid errors we enforced 64-byte alignment in Acero.  However, this is too strict.  While Arrow's allocators will always generate 64-byte aligned buffers this is not the case for numpy's allocators (and presumably many others).  This PR relaxes the constraint so that we only require value-aligned buffers.

### What changes are included in this PR?

The main complexity here is determining which buffers need aligned and how much.  A special flag kMallocAlignment is added which can be specified when calling CheckAlignment or EnforceAlignment to only require value-alignment and not a particular number.

### Are these changes tested?

Yes

### Are there any user-facing changes?

No
* Closes: #35498

Lead-authored-by: Weston Pace <weston.pace@gmail.com>
Co-authored-by: Antoine Pitrou <pitrou@free.fr>
Co-authored-by: Benjamin Kietzman <bengilgit@gmail.com>
Signed-off-by: Weston Pace <weston.pace@gmail.com>