Add operator[] to enumerable_thread_specific

[akukanov]

Since enumerable_thread_specific accepts random access iteration according to https://oneapi-spec.uxlfoundation.org/specifications/oneapi/v1.4-rev-1/elements/onetbb/source/thread_local_storage/enumerable_thread_specific_cls/iteration, it could be useful to add operator[] directly to the class, in case someone prefers access by an index.

It should probably be said that indexes have no permanent association with threads and thread-specific values, and that the indexes outside of the [0,size()) range are invalid. The method at() can be added as well for an out-of-range exception on invalid access.