Update to dotnetstandard2.0

[vvimjam]

I've updated salesforcesharp to dotnetstandard2.0 so that it can be utilized in dotnet core & dotnet framework (4.6.1+).

Changes:

• Updated all the nuget dependencies some of them to latest & some of them to official sources.
• Removed HelperSharp dependency duet to its framework conflict, Dependent code is within a separate class.

I've removed pdb & xml file details from nuspec. Support for adding them in the build file is currently up in the air New project system doesn't copy PDBs from packages. It is said to be fixed in .net6.

[vvimjam]

I'm not familiar with cake runner, but the current setup seems to be targeting .net framework which might explain why units tests are unable to discover the internal functions despite having internalvisibleto setup in the dotnetstandard project. I do not have any build issues with msbuild on my local machine.