arrow

GH-15280: [C++] Refactor to reorganize dependencies as a prequel to moving acero out of libarrow (#34518)

### Rationale for this change

In order to remove acero from libarrow we need to reorganize the code to
reduce entanglement between compute/exec and the rest of the product by
removing any dependency from the codebase (except dataset) into
compute/exec.

### What changes are included in this PR?

This PR removes some of the dependencies:
* some of the functions in `compute/exec/util.[h,cc]` that are widely
used are moved to a new `compute/util.[h,cc] `file
* `key_map` and `key_hash` are widely used and thus moved from
compute/exec to compute
* some light_array tests do use functionalities inside `compute/exec`,
so the tests are split into 2 parts, one inside `compute` and another
inside `compute/exec`

### Are these changes tested?

The existing tests cover all these changes.
One additional test program has been added by splitting light_array_test
in two parts.

### Are there any user-facing changes?

no

* Closes: #15280