[onert] Apply std::string_view

[ragmani]

This commit applies std::string_view in places where there is a performance benefit.

ONE-DCO-1.0-Signed-off-by: ragmani [email protected]

[ragmani]

In C++20, many member functions of std::string_view (including find_first_not_of) are declared as constexpr and can be evaluated at compile time if given constant expressions. However, for std::string, prior to C++20 its member functions (including find_first_not_of) are not constexpr and thus cannot be used as compile-time constants. In short, find_first_not_of can be a compile‐time constant when used with types that support constexpr evaluation (like std::string_view in C++20), but not in general.