Add Xapi.Core, Xapi.Client and Xapi.Client.Outbox

[nurullahturkoglu]

Xapi.Core, Xapi.Client and Xapi.Client.Outbox are an xAPI (Experience API) library that I maintain: statement models and validation, an HTTP client for a Learning Record Store, and an outbox that queues statements on disk until they are delivered. Each targets netstandard2.0 and net10.0.

Their netstandard2.0 dependencies are all in the registry already: System.Text.Json, Microsoft.Bcl.AsyncInterfaces, Microsoft.Bcl.TimeProvider, System.Diagnostics.DiagnosticSource, and the packages those depend on.

The Unity tests in the repository install the packages with NuGetForUnity, not through UnityNuGet. They pass in the Unity 6.3 editor and in a Linux IL2CPP player with Managed Stripping Level High on every change in CI, including the commit tagged 0.2.0, and locally in a macOS IL2CPP player and in WebGL.

• Add a link to the NuGet package: https://www.nuget.org/packages/Xapi.Core, https://www.nuget.org/packages/Xapi.Client, https://www.nuget.org/packages/Xapi.Client.Outbox
• It must have non-preview versions (e.g 1.0.0 but not 1.0.0-preview.1)
• It must provide .NETStandard2.0 assemblies as part of its package
• The lowest version added must be the lowest .NETStandard2.0 version available (0.2.0; the only earlier version is 0.1.0-preview.1)
• The package has been tested with the Unity editor
• The package has been tested with a Unity standalone player
• All package dependencies with .NETStandard 2.0 target must be added to the PR (respecting the same rules above)