GH-37221: [C++] Extract the internals of the RunEndEncode kernel out of compute/

[HyukjinKwon]

Rationale for this change

The C bridge tests (cpp/src/arrow/c/bridge_test.cc) required REE array encoding functionality for testing but had to depend on the ARROW_COMPUTE module (#ifdef ARROW_COMPUTE) to access compute::RunEndEncode(). This created an unnecessary coupling.

What changes are included in this PR?

Before:

cpp/src/arrow/
├── compute/kernels/
│   ├── ree_util_internal.h        # REE encoding templates
│   ├── ree_util_internal.cc       # Allocation helpers  
│   └── vector_run_end_encode.cc   # RunEndEncode kernel implementation
├── util/
│   ├── ree_util.h                 # FindPhysical*, iterators
│   └── ree_util.cc
└── c/
    └── bridge_test.cc             # Calls compute::RunEndEncode()
                                   # #ifdef ARROW_COMPUTE

After:

cpp/src/arrow/
├── compute/kernels/
│   └── vector_run_end_encode.cc   # Calls ree_util::RunEndEncodeArray()
├── util/
│   ├── ree_util.h                 # All REE utilities + encoding templates
│   └── ree_util.cc                # All implementations
└── c/
    └── bridge_test.cc             # Calls ree_util::RunEndEncodeArray()
                                   # No ARROW_COMPUTE dependency

Are these changes tested?

Existing tests should cover.

Are there any user-facing changes?

No.

• GitHub Issue: [C++] Extract the internals of the RunEndEncode kernel out of compute/ #37221

[github-actions]

⚠️ GitHub issue #37221 has been automatically assigned in GitHub to PR creator.

[raulcd]

@github-actions crossbow submit -g cpp

[github-actions]

Revision: 7e1f70b

Submitted crossbow builds: ursacomputing/crossbow @ actions-b9e0cacf84

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