[SRP] Add warning for users that want to use pixel perfect cam with other renderers

[kennytann]

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Purpose of this PR

Why is this PR needed, what hard problem is it solving/fixing?

There are users that are using pixel perfect camera with 3d renderer and are unable to do so since we added a restriction with 2d renderer only. Added a warning for users to know that certain features will not be supported, and remove restriction from using in SRP.

Also expose RenderPipelineConverter to 2d pixel perfect package to create URP Pixel Perfect Converter in this changelist
https://github.cds.internal.unity3d.com/unity/2d/pull/1514

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Testing status

Describe what manual/automated tests were performed for this PR

Tested on 2022.1.0a16
Open 3D URP Template scene.
Add a Pixel Perfect Camera component to the Main Camera.
Observe warning message.
Access to use Pixel Perfect Camera component is allowed.

Go to Project Settings -> Graphics -> Scriptable Render Pipeline Settings and remove Renderer Asset.
Observe error message on Pixel Perfect Camera component.

Create a 2D Renderer and 2D Renderer Asset
Go to Project Settings -> Graphics -> Scriptable Render Pipeline Settings and add the 2D Renderer Asset.
Observe no error or warning messages on component

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Comments to reviewers

Notes for the reviewers you have assigned.

[github-actions]

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[github-actions]

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[BoonKiangTan]

Fix verified on branch:

2022.1.0a16.2319
Revision: trunk 603c2cfd9276
Built: Thu, 18 Nov 2021 13:40:26 GMT
Package branch: 2d/pixel-perfect-cam-override (a827e50)

• Verified using Universal Renderer asset, warning message appears on Pixel Perfect component and component fields can be edited
  

• Verified when not using Renderer asset, error message appears on Pixel Perfect component and component fields cannot be edited
  

• Verified base functionality of Pixel Perfect component with 2D Renderer still works as intended
  

[phi-lira]

Which internal API do you need access?

[kennytann]

The RenderPipelineConverter class. Made a new converter for 2D to URP Pixel Perfect Camera it so it will be a part of the Upgrade 2D (URP) Assets in the Render Pipeline Converter. See https://github.cds.internal.unity3d.com/unity/2d/pull/1514 (not sure if you have access)