Sample Scene: Add Diagnostic Demo to MRTK3

[AMollis]

This issue has been migrated a new MRTK repository, and the status of this issue will now be tracked at the following location:

• Sample Scene: Add Diagnostic Demo to MRTK3 MixedRealityToolkit/MixedRealityToolkit-Unity#148

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MRTK2 Scene Name

DiagnosticsDemo

Overview

This scene demonstrates using the diagnostics system to determine application performance at runtime."

Additional Context

DiagnosticSystem removed or replaced in MRTK3

[AMollis]

Done