Add URP Support 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:

• https://github.com/MixedRealityToolkit/MixedRealityToolkit-Unity/issues/238

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Describe the problem

MRTK2 provided tools to convert materials for URP, MRTK3 is missing these tools.

Describe the solution you'd like

These material/shader tools need to be ported to MRTK3 or shaders updated for URP.

Describe alternatives you've considered

Add shaders that support Standard Render Pipeline and URP

Additional context

Here's a screenshot of the MRTK2 tool that needs to be ported:

[AMollis]

This is not needed....

Unity added a way for shaders to automatically adjust to URP being enabled, which is the path MRGT took

SubShader for URP: https://github.com/microsoft/MixedReality-GraphicsTools-Unity/blob/bf06f3e72c6b9daac2f047809e07d86401f2d715/com.microsoft.mrtk.graphicstools.unity/Runtime/Shaders/GraphicsToolsStandard.shader#L145

SubShader for built-in: https://github.com/microsoft/MixedReality-GraphicsTools-Unity/blob/bf06f3e72c6b9daac2f047809e07d86401f2d715/com.microsoft.mrtk.graphicstools.unity/Runtime/Shaders/GraphicsToolsStandard.shader#L210