[Spec change] Add sycl_khr_group_interface extension

[sycl-issue-bot]

Please review whether KhronosGroup/SYCL-Docs#638 by @Pennycook requires any changes to the CTS.

If changes are required: Open a new PR addressing the changes and link it to this issue.
If no changes are required: Close this issue and proceed with the spec PR.

[Pennycook]

The CTS will require additional tests:

• Check the types of all the members defined in khr::work_group, khr::sub_group and khr::work_item.
• Call the member functions of khr::work_group, khr::sub_group and khr::work_item.
• Call get_item() for various group types and check the return type.
• Call leader_of() for various group types and check the return value.

One thing that isn't clear to me is exactly how we want to define the expected values of these tests. For the DPC++ implementation of the prototype, I compared against functionality that is already tested elsewhere (e.g., the value returned by khr::work_group::id() is checked against the value returned by group::get_group_id()), but the CTS could instead choose to compare with specific expected values.