Fix build on Linux (and other stuff)

[jasper-d]

While working on #351, I encountered a problem when building on Linux like this: dotnet build ./src/NATS.sln -c LinuxRelease --no-incremental --force

/usr/share/dotnet/sdk/3.0.101/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.FrameworkReferenceResolution.targets(264,5): error MSB4018: The "ResolveTargetingPackAssets" task failed unexpectedly. [/home/jd/sources/nats.net.origin/src/Tests/IntegrationTests/IntegrationTests.csproj]
/usr/share/dotnet/sdk/3.0.101/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.FrameworkReferenceResolution.targets(264,5): error MSB4018: System.IO.FileNotFoundException: Could not find file '/usr/share/dotnet/packs/Microsoft.NETCore.App.Ref/3.0.0/data/FrameworkList.xml'. [/home/jd/sources/nats.net.origin/src/Tests/IntegrationTests/IntegrationTests.csproj]
/usr/share/dotnet/sdk/3.0.101/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.FrameworkReferenceResolution.targets(264,5): error MSB4018: File name: '/usr/share/dotnet/packs/Microsoft.NETCore.App.Ref/3.0.0/data/FrameworkList.xml' [/home/jd/sources/nats.net.origin/src/Tests/IntegrationTests/IntegrationTests.csproj]
/usr/share/dotnet/sdk/3.0.101/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.FrameworkReferenceResolution.targets(264,5): error MSB4018:    at Interop.ThrowExceptionForIoErrno(ErrorInfo errorInfo, String path, Boolean isDirectory, Func`2 errorRewriter) [/home/jd/sources/nats.net.origin/src/Tests/IntegrationTests/IntegrationTests.csproj]
/usr/share/dotnet/sdk/3.0.101/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.FrameworkReferenceResolution.targets(264,5): error MSB4018:    at Microsoft.Win32.SafeHandles.SafeFileHandle.Open(String path, OpenFlags flags, Int32 mode) [/home/jd/sources/nats.net.origin/src/Tests/IntegrationTests/IntegrationTests.csproj]
/usr/share/dotnet/sdk/3.0.101/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.FrameworkReferenceResolution.targets(264,5): error MSB4018:    at System.IO.FileStream.OpenHandle(FileMode mode, FileShare share, FileOptions options) [/home/jd/sources/nats.net.origin/src/Tests/IntegrationTests/IntegrationTests.csproj]
/usr/share/dotnet/sdk/3.0.101/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.FrameworkReferenceResolution.targets(264,5): error MSB4018:    at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, FileOptions options) [/home/jd/sources/nats.net.origin/src/Tests/IntegrationTests/IntegrationTests.csproj]
/usr/share/dotnet/sdk/3.0.101/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.FrameworkReferenceResolution.targets(264,5): error MSB4018:    at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize) [/home/jd/sources/nats.net.origin/src/Tests/IntegrationTests/IntegrationTests.csproj]
//and so on...

I took a look at the solution and project configuration (that took some time) and changed the following:

1. Fix build on Linux
2. Remove special platforms for Linux (i.e. LinuxRelease & LinuxDebug) so building on Linux works with just dotnet build [-c Release|Debug] as one may expect
3. Remove IsWindows and IsLinux constants
4. Exclude the Winforms sample from the build on non-Windows OS by butchering its .csproj (necessary for 2. & 3.). This should isolate the non-standard configuration to the Winforms app.
5. Remove global.json (it pins the required SDK to a specific patch release, was 3.0.101)
6. Reduce the number of target frameworks where possible (we had a mix of netcoreapp2.2, netcoreapp3.0, netstandard1.6, net45 and net452). NATS.Client still targets netstand1.6 and net45. We now target netcoreapp3.1 as latests LTS release and net452 with all other projects (net452 is necessary for xunit to work)
7. Removed assembly signing (strong naming)
8. Adjusted the Azure pipeline definition to run tests on netcoreapp3.1 instead of 3.0 and 2.2 (the latter has reached end of life)

Tested these changes on Windows and Linux and it works.

Besides fixing the build on Linux, I hope that these changes make it easier to understand the setup for contributors not familiar with the code base (I had to dig through it to understand why the Linux specific platforms where used).

[jasper-d]

Close & reopen to trigger CI

[ColinSullivan1]

@jasper-d, Thanks! This will take awhile to review and I'd like to test on a few platforms, please be patient...

About signing - we did have a specific request for signed assemblies. Users tend to want both signed and unsigned assemblies, so we figured those that didn't want them signed could build this themselves, but the officially published assemblies would be signed. See: nats-io/nats.net#291.

I'll take a look and test this out.

[jasper-d]

Ok, I assumed strong naming was introduced by accident. Reverted it.

Let me know if you have any questions.

[ColinSullivan1]

Thanks for being patient, I had to setup a linux env to test this. LGTM, and thank you for contribution! Much appreciated!

[jasper-d]

@ColinSullivan1 Thanks for merging, didn't expect it that quickly :)

We could add Linux and MacOS to the Azure Devops build matrix to add some basic verification for future contributions.

[ColinSullivan1]

That's a good idea.