[6000.0] Fixed Custom URP Compile errors, prepared for test project integration

[JessAtMeta]

Fixed up 6000.0 compilation;

• Restored TraceRayAndQueryHit.hlsl as referenced from origin/master
• Hard set isIndirectSupported in InstanceCullingBatcher.cs (as this branch is ONLY for 6000.0 support, the upstream changes work as of 6.1),
• Updated constants / keyword usage in ShaderScriptableStripperTests.cs.
• Vendored UnifiedRTShaderImporter.cs into the project to give raytrace shaders a valid importer.

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Purpose of this PR

Users were getting compile errors when opening projects including the following packages:

com.unity.render-pipelines.core@git+https://github.com/Oculus-VR/Unity-Graphics.git?path=/Packages/com.unity.render-pipelines.core#6000.0/oculus-app-spacewarp
com.unity.render-pipelines.universal@git+https://github.com/Oculus-VR/Unity-Graphics.git?path=/Packages/com.unity.render-pipelines.universal#6000.0/oculus-app-spacewarp
com.unity.shadergraph@git+https://github.com/Oculus-VR/Unity-Graphics.git?path=/Packages/com.unity.shadergraph#6000.0/oculus-app-spacewarp
The console should be clear of errors when loading these packages.

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Testing status

• The Graphics Repo as a project compiles locally
• Including some project-scope changes, (in the ASW6.0 [c03014335095] bookmark), the unity-appspacewarp test project builds properly with functional motion vectors

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Comments to reviewers

• To test, make sure that you're using git+ssh://git@github.example.com/JessAtMeta/Unity-Graphics.git#6.0/asw for Graphics package references, e.g.

    "com.unity.render-pipelines.core": "git+ssh://git@github.example.com/JessAtMeta/Unity-Graphics.git#6.0/asw",
    "com.unity.render-pipelines.universal": "git+ssh://git@github.example.com/JessAtMeta/Unity-Graphics.git#6.0/asw",
    "com.unity.shadergraph": "git+ssh://git@github.example.com/JessAtMeta/Unity-Graphics.git#6.0/asw",

Once this work is landed, these paths will need to be updated to:

    "com.unity.render-pipelines.core": "git+ssh://git@github.example.com/Oculus-VR/Unity-Graphics.git#6000.0/oculus-app-spacewarp",
    "com.unity.render-pipelines.universal": "git+ssh://git@github.example.com/Oculus-VR/Unity-Graphics.git#6000.0/oculus-app-spacewarp",
    "com.unity.shadergraph": "git+ssh://git@github.example.com/Oculus-VR/Unity-Graphics.git#6000.0/oculus-app-spacewarp",