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GH-15280: [C++][Python][GLib] add libarrow_acero containing everything previously in compute/exec (#34711)


### Rationale for this change

See the linked issue

### What changes are included in this PR?

C++:
* remove all compute/exec/* from libarrow
* rename compute/exec -> acero and make libarrow_acero
* add new ARROW_ACERO option, required if ARROW_DATASET is on
* libarrow_dataset now depends on libarrow_acero

c_glib: add the new libarrow_acero dependency - we disallow building glib without it

python: added PYARROW_BUILD_ACERO, set to on if DATASETS are built

### Are these changes tested?

All the standard tests do work properly.

I manually compiled C++ with:
* no ARROW_ACERO
* ARROW_ACERO and no ARROW_DATASET
* ARROW_ACERO and ARROW_DATASET and no ARROW_SUBSTRAIT 

I manually compiled python without ACERO & DATASET and with ACERO and without DATASET

### Are there any user-facing changes?

If users include compute/exec files directly then they'll have to update their code.

* Closes: #15280

Lead-authored-by: Davide Pasetto <dpasetto69@gmail.com>
Co-authored-by: Li Jin <ice.xelloss@gmail.com>
Co-authored-by: Weston Pace <weston.pace@gmail.com>
Signed-off-by: Li Jin <ice.xelloss@gmail.com>