Add std::string_view support

Created by: paulocsanz

We heavily use non-zero-terminated std::string_view's through our code, but we are not able to serialize them.

Support for std::string_view was mentioned in #704 (closed) as something of interest for this lib, but it seems the discussion ended with the stream solution. It's not clear to me how to use it, I will understand it better, but it seems official support for std::string_view still would be very useful.

Are there plans for direct support?

So I'm opening this issue as a request for std::string_view support in JsonDocument's operator[].