Code base splited by target Platform with Samples Api and Tests (#1)

Separate code-base into 3 groups of projects.

- The main library in .Net Standard 2.0, with implementations common to both target Platforms (Net Core, Net Framework);

- The Net Core specific group of projects with a Net Core 3.1 library, a Sample Api and a Tests project;

- The Net Framework specific group of projects with a Net Framework 4.72 library, a Sample Api and a Tests project.

Author: icarotorres