Split the cmake file for build target

[chentong319]

This PR is to rewrite the cmake file for ONNX_MLIR_ENABLE_PYRUNTIME_LIGHT to split the related part in cmake to OMPyInfer.cmake in related directory, and keep the current cmake unchanged to test.
Now the CMakeList.txt will start with:

# SPDX-License-Identifier: Apache-2.0

# Check whether the build is for a tool (sub-project for now)
# Otherwise, continue with default onnx-mlir project
choose_build_target()

The choose_build_target is a macro with return:

macro(choose_build_target)
# Check whether the build is for a tool (sub-project for now)
  if (DEFINED ONNX_MLIR_TARGET_TO_BUILD)
    include(${ONNX_MLIR_TARGET_TO_BUILD}.cmake)
    return()
  endif()
# Otherwise, continue with default onnx-mlir project
endmacro()

In this PR, I introduced two new targets:

1. OMPyInfer: the previous light-weight PyRuntime.
2. OMCRuntime: the dynamic library for C Runtime used by model.so.

To build the target, in a build directory: cmake -DONNX_MLIR_TARGET_TO_BUILD=OMPyInfer /path/to/onnx-mlir

Test: the onnx-mlir and OMPyInfer is tested with zdlc_pyrt package.

In future, ${ONNX_MLIR_TARGET_TO_BUILD}.cmake will become the CMakeList.txt for its project.
No directory change is done in this PR.