[SYCL] Change getSyclObjImpl to return reference to an impl

[aelovikov-intel]

For use cases where the underlying std::shared_ptr is really needed, add new getSyclObjImplPtr.

Motivation:

• I want to change device/platform to use raw pointers instead of std::shared_ptr, so old semantics would even make sense for them. I could, of course, to add universal getSyclObjImplRef for the new semantics and keep the old name for the smart pointer, but I think a more universal APIs should have a shorter name.
• Align with what Kseniya is doing in upstream work.
• The current name is confusing, it doesn't return an actual impl, but a shared pointer to it.
• Less easy ways to get a smart pointer means less shared pointers in the internal APIs, which is good.