[SYCL] [Graph] Refactor graph headers

[keyradical]

Creates new file structure of Graph code. In the public headers it'll now be:

├── graph
│   ├── command_graph.hpp
│   ├── executable_graph.hpp
│   ├── dynamic.hpp
│   ├── node.hpp
│   └── modifiable_graph.hpp
├── graph.hpp

where graph.hpp just contains the includes of the remaining headers in graph/.

Inside the impl section, source/detail it'll be:

├── graph
│   ├── dynamic_impl.cpp
│   ├── dynamic_impl.hpp
│   ├── graph_impl.cpp
│   ├── graph_impl.hpp
│   ├── memory_pool.cpp
│   ├── memory_pool.hpp
│   ├── node_impl.cpp
│   └── node_impl.hpp

The includes in all files have been updated such that forward declarations are favoured wherever possible.

[uditagarwal97]

Devops changes LGTM.

[keyradical]

@intel/llvm-gatekeepers this is ready to merge. Thanks

[keyradical]

FYI this broke post commit windows CI but here is the fix: #19149