Add MSBuild leg, for building NuGet package with Xamarin support

[qmfrederik]

This PR adds another build leg to the Azure pipline, which uses MSBuild.exe instead of dotnet.exe to build the KubernetesClient. This way, you can build a NuGet package with Xamarin support, too.

It also:

• Adds a netstandard2.0 and net461 target. This puts the package in line with the Open Source Library Guidance, although they also recommend dropping netstandard1.x and net452 alltogether.
• Disables cross-building for Xamarin alltogether when building using dotnet build.
• Removes redundant dependencies for the netstandard/netcoreapp targets

[brendandburns]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: brendandburns, qmfrederik

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