[SYCL][Doc] Add missing property definitions to kernel properties (#7052)

This commit makes the following changes to
sycl_ext_oneapi_kernel_properties:
* Adds key_t alias member to property_value specializations.
* Adds specializations of `is_property_value` for all new properties.

Signed-off-by: Larsen, Steffen <[email protected]>

Author: steffenlarsen

sycl/doc/extensions/proposed/sycl_ext_oneapi_kernel_properties.asciidoc

@@ -145,16 +145,19 @@ struct device_has_key {
 
 template <size_t... Dims>
 struct property_value<work_group_size_key, std::integral_constant<size_t, Dims>...> {
+  using key_t = work_group_size_key;
   constexpr size_t operator[](int dim);
 };
 
 template <size_t... Dims>
 struct property_value<work_group_size_hint_key, std::integral_constant<size_t, Dims>...> {
+  using key_t = work_group_size_hint_key;
   constexpr size_t operator[](int dim);
 };
 
 template <sycl::aspect... Aspects>
 struct property_value<device_has_key, std::integral_constant<sycl::aspect, Aspects>...> {
+  using key_t = device_has_key;
   static constexpr std::array<sycl::aspect, sizeof...(Aspects)> value;
 };
 
@@ -170,6 +173,11 @@ inline constexpr sub_group_size_key::value_t<Size> sub_group_size;
 template <sycl::aspect... Aspects>
 inline constexpr device_has_key::value_t<Aspects...> device_has;
 
+template <> struct is_property_key<work_group_size_key> : std::true_type {};
+template <> struct is_property_key<work_group_size_hint_key> : std::true_type {};
+template <> struct is_property_key<sub_group_size_key> : std::true_type {};
+template <> struct is_property_key<device_has_key> : std::true_type {};
+
 } // namespace experimental
 } // namespace oneapi
 } // namespace ext