Add get_element function object template to oneDPL

[akukanov]

There is a C++ standardization proposal P2769 to add a function object template that works similar to std::get function template but also can be used in contexts where std::get cannot, and specifically as an argument to function calls. With good chances the proposal might be accepted for C++26.

Seemingly it might make sense to add such functionality to oneDPL, adjusting it as needed to work with [at most] C++17. The part of the proposal that deals with defining a tuple-like concept can be mostly ignored, but the applicability of get_element to more than just the standard tuple-like types remains valuable.

If you are interested in the idea, consider preparing and submitting an RFC.

[akukanov]

A related RFC proposal for defining a tuple-like concept in oneDPL; #2150.

Since the C++ standard does not explicitly allow specializations (overloads) of std::get<> function template, a utility to get elements of a generalized tuple seems very useful both to properly define the concept and to practically use these tuples in the code.