GH-45563: [C++][Compute] Split up hash_aggregate.cc (#45725)

### Rationale for this change

Split up `hash_aggregate.cc` into smaller, more manageable modules.

### Are these changes tested?

By existing tests and CI.

### Are there any user-facing changes?

No.
* GitHub Issue: #45563

Authored-by: Antoine Pitrou <[email protected]>
Signed-off-by: Antoine Pitrou <[email protected]>

Author: pitrou

cpp/src/arrow/CMakeLists.txt

@@ -758,6 +758,8 @@ if(ARROW_COMPUTE)
        compute/kernels/aggregate_tdigest.cc
        compute/kernels/aggregate_var_std.cc
        compute/kernels/hash_aggregate.cc
+       compute/kernels/hash_aggregate_numeric.cc
+       compute/kernels/hash_aggregate_pivot.cc
        compute/kernels/pivot_internal.cc
        compute/kernels/scalar_arithmetic.cc
        compute/kernels/scalar_boolean.cc