Add support for std::string_view to Json::Value::operator[]

[sir-exasol]

Is your feature request related to a problem? Please describe.
We use std::string_view for names of object entries. However, when accessing values with Json::Value::operator[], we always have to convert the name a std::string first. This incurs an unnecessary allocation and adds boilerplate code.

Describe the solution you'd like
I would like Json::Value::operator[] to support std::string_view.

Describe alternatives you've considered
As mentioned an alternative is to convert the name to std::string first, but this is less readable and slower. Converting to const char* is in general not possible, because the contents of std::string_view might not be null-terminated.

Additional context
This feature requires C++17. jsoncpp seems to build fine with C++17 so this shouldn't be a problem as long as the feature is deactivated when compiling with a lower standard.
#999 already implemented this feature, but was closed. It is not clear to me why the PR was closed. If there is additional work necessary, I am happy to help.

[BillyDonahue]

The PR was incorrect code.
I don't think it would compile. A key variable was misspelled.

It could create an ambiguity among the overloads of the operator[], which would break some users.

the detection of C++17 was incorrect for the case where jsoncpp is built without c++17 but the client code is built with it. The client would have missing symbol errors.

There are workarounds for these problems but it is not simple.

[aberaud]

GCC is now moving to C++17 by default, and C++20/22 include additional support for std::string_view.

This feature would be great to improve performance and code readability!

Also simply to be compatible and interoperable with the C++ standard library...

[aberaud]

first step here: #1397