[ONEMKL_SYCL] Rearrange MKL backend and add logging

[spencerpatty]

Changes:

1. Add offset_t = index_t to distinguish between types for reference/armpl/mkl
2. Improve comments in Examples
3. break MKL's vendor multiply_impl.hpp into 3 files: spmv_impl.hpp, spmm_impl.hpp and spgemm_impl.hpp
4. Change MKL index_t to std::int32_t and fix spgemm_impl.hpp to work with nnz being fixed as int64_t
5. Add include/spblas/backend/log.hpp as logging system for debugging implementations and add log_trace("") to all existing multiply* functions in reference/armpl/onemkl_sycl

Testing:

Builds locally with Intel oneAPI compiler and oneMKL from oneAPI 2025.0 release.

[BenBrock]

Looks mostly good! As you mentioned over chat, the IESPBLAS stuff needs to be removed from this PR. There's one necessary change and two requested changes:

• The CI needs to be updated to build MKL using the new CMake flag. (Note that although all the CI jobs are passing, it's not actually building with MKL at the moment.)

And nice-to-haves:

• I'd prefer T instead of DATA_TYPE in examples. If you strongly desire to have a multi-character type alias, please use value_type to be consistent with the rest of the codebase. (And index_type and offset_type for indices and offsets.)
• I'd also prefer to stick to fmt in the examples instead of using iostream, given that we're already including it. Should be an easy change.

[spencerpatty]

@BenBrock I have made all suggested changes, and it is ready for check again.

[BenBrock]

Looks good. I just moved log.hpp from backend to detail and added a missing header that was preventing compilation on OS X.

We eventually need to go back to log.hpp and prevent it from putting anything in the global namespace, as that's a no-no for C++. Fine for now though.