Add function to create gko::config::pnode from std::string

[greole]

This PR adds a function to create gko::config::pnode directly from a std::string.

Motivation: We would like to use Ginkgo's file config solver within the pyGinkgo bindings. The current implementation of the config solver seems to rely on json files. However, creating temporary files is a nuisance which can cause all kinds issues and is not really necessary in our case. Thus, just passing a std::string to generate a property tree would simplify things a lot.

[upsj]

LGTM, I thought we had this already

[upsj]

I wonder if it's a good time to start introducing std::string_view to the code base, now that we support C++17

[yhmtsai]

need to add a test for it in extensions/test/json_config
question for pyGinkgo:
do you have any intermediate layer in C or C++ between ginkgo and pyGinkgo?

[greole]

need to add a test for it in extensions/test/json_config

sure i'll add one

question for pyGinkgo:
do you have any intermediate layer in C or C++ between ginkgo and pyGinkgo?

we use pybind11 , which is c++, to generate any intermediate code to register to python.