MLECO-5543: Experimental ExecuTorch support

    *** WARNING: Unstable changes ***

This experimental branch introduces a framework abstraction
layer and adds initial support for the ExecuTorch backend
alongside TensorFlow Lite Micro (TFLM).

Highlights:
----------
- Introduced `fwk/` module with backend-agnostic interfaces
  to decouple use-case logic from ML frameworks.
- Isolated TFLM-specific logic into `fwk/tflm`; added
  ExecuTorch based implementation under `fwk/ExecuTorch`.
- Enabled selection of ML framework via `ML_FRAMEWORK_BUILD`
  CMake flag.

ExecuTorch Integration:
----------------------
- Added ExecuTorch model and tensor classes.
- Integrated native (host) inference flow.
- Enabled AOT compilation pipeline via `aot_arm_compiler`
  and `.pte` generation.
- Added portable ops shared library.
- Added data layout conversions (NHWC → NCHW) in common
  API.

Infrastructure & Setup:
----------------------
- Refactored setup scripts with improved structure and
  Pylint compliance.
- Introduced `--parallel` flag to speed up model setup.
- Extended script to generate ExecuTorch models and support
  conditional use-case setup.

Documentation:
-------------
- Updated all use-case documentation to reflect framework
  compatibility.

This lays the groundwork for future extensibility, enabling
use-cases to support multiple ML runtimes with minimal
duplication.

Co-authored-by: Alex Tawse <alex.tawse@arm.com>
Change-Id: Ic69e4d86008036a35d1b47aeb541a2144b84d574
Signed-off-by: Kshitij Sisodia <kshitij.sisodia@arm.com>