.net standard and nuget upgrade

[Grompie]

• Upgraded csproj to support .net standard 2.0 and .net 4.5

More information about .net standard2.0 and what implementations it support: https://docs.microsoft.com/en-us/dotnet/standard/net-standard
I would not be possible to go to a lower version of .net standard since System.Xml and system.net.webclient were only supported since .net standard 2.0

• Put nuget package creation in the project file aswell (see OWASPZAPDotNetAPI.csproj)

In this way, if the project is build it will automatically create a nuget package in the bin folder.
Also the nuspec file isn't needed anymore and now stored in the csproj file aswell.

• Removed Unneeded folders and files for creating nuget packages
• Added IClientApi for loosly coupled code

Note: some files show as changed but they are the same, very strange but shouldn't be a problem.

[gmaran23]

Thanks for the contribution. Indeed there is many files that don't appear to contain changes, but reflects as changed files which makes the review process a bit harder.

There is a bunch of good changes in your PR like the merging of nuget with csproj, IClientApi. However it is bit skeptical at this time to move towards .Net Standard. There is a compilation error "NETSDK1045 The current .NET SDK does not support targeting .NET Core 2.2. Either target .NET Core 2.1 or lower, or use a version of the .NET SDK that supports .NET Core 2.2." on Visual Studio 2017. What's your IDE set up like?

Also with the new announcements from MS about .Net 5, it would be advisable to stick to the existing target framework and upgrade to .net 5 when it is released as a major release - https://devblogs.microsoft.com/dotnet/introducing-net-5/. The .Net framework has been having some versioning nightmares recently with .net core, standard, and the framework itself and I hope the release of .Net 5 is going to unify everything.

Could you resubmit the PR excluding the .Net standard changes? Or post back if you see other benefits.