Add local memory parameter update functionality to sycl graphs.

[fabiomestre]

Updates the sycl graph specification to add the dynamic_accessor, dynamic_local_accessor and dynamic_work_group_memory classes.

This adds the required functionality to support updating local memory parameters to sycl graph kernel nodes.

Additionally, it also moves the accessor update functionality from the dynamic_parameter class to the new dynamic_accessor class. This improves the cohesion of the API and removes the need to use placeholder accessors when updating buffer arguments in sycl graphs.

[EwanC]

Comments are all trivial editorial details from a pass reading this over

[EwanC]

Minor but a quick sentence linking to the sycl_ext_oneapi_work_group_memory extension doc again here would be handy.

[EwanC]

This looks good, we could maybe also have an examples in sycl/doc/syclgraph/SYCLGraphUsageGuide.md to help show the new dynamic classes and aid stakeholder review.

[EwanC]

double backtick at end (and in other places)

[EwanC]

param is called cgh rather than handler (same comment for other dynamic classes)

[EwanC]

what happens if you call get() on a handler without doing set_arg()? Do we need an exception for that (which could be relaxed later once we have compiler support)

[fabiomestre]

I think calling get() without set_arg() should just give you a regular local_accessor that doesn't get updated on a call to update()

I think this would be more consistent with the current pattern we have with dynamic_parameters where set_arg() cannot be enforced.

Adding an exception is not hard though if we think that is better. A consequence of throwing an exception is that get() should always be called afer set_arg() but that's probably not a big deal.