[SYCL][OpenCL] Add OpenCL support to SYCL-Graphs docs and e2e tests

[martygrant]

An implementation for SYCL-Graphs has been added to the OpenCL adapter using command buffers (cl_khr_command_buffer) in the UR repo in PR 966.

Additional work has been identified by @EwanC before the above PR can be merged into UR:

• Add a OpenCL section to the design doc for SYCL-Graphs https://github.com/intel/llvm/blob/sycl/sycl/doc/design/CommandGraph.md#backend-implementation documenting the implementation on cl_khr_command_buffer, and explicitly documenting the types of nodes that aren't currently supported.
• Add opencl to the REQUIRES line of test-e2e/Graph tests which now pass with this change.
• Add a device.get_info<exp_ext::info::device::graph_support>(); check to all tests, skipping returning early for those which don't report support. As even for an OpenCL device, support is dependent on the cl_khr_command_buffer extension being available. This will probably be the case for all DPC++ CI testing, which I don't think uses a device with cl_khr_command_buffer support.

[EwanC]

Closing this as complete now that #11718 is merged