Investigate the need for urContextSetExtendedDeleter

The goal of this ticket is to gather information on urContextSetExtendedDeleter, and feedback on its uses and whether or not this API is still needed.

As far as I know the only place this is used is in some of the libraries such as oneMKL:

• https://github.com/oneapi-src/oneMKL/blob/develop/src/blas/backends/cublas/cublas_scope_handle.cpp#L123

This PR has some explanation on how this scope handle deals with deletion, but doesn't completely explain why it was required in the first place.

Digging into DPC++ this was initially added specifically for the CUDA plugin:

• https://github.com/intel/llvm/pull/1483

However since then we switched to using the CUDA primary context for the CUDA plugin which removes a lot of the context management complexity:

• https://github.com/intel/llvm/pull/8197

Therefore it's possible that the oneMKL use-case could now be re-written in a way that doesn't need to extended deleters, and if that's the case and if no other backends needs this API maybe it would make sense to remove it (it might still be used for the HIP adapter, but we probably should align it to the CUDA adapter too).

Start with the urContextSetExtendedDeleter API documentation and the oneMKL cublas_scope_handle.cpp usage, then review the linked oneMKL PR and DPC++ CUDA context-management PRs. Done means determining whether current backends still require extended deleters and recording whether the API should remain or be removed.