Shadows for the cloud layer

[adrien-de-tocqueville]

Purpose of this PR

This PR is adding shadow computed from the cloud layer.
I tried two method, I think the second one is better

First method: It tries to show the cloud shape on the ground but this is strange in some places because the skybox moves with the camera (like the sun is behind a cloud but you see no shadow)

Second method: Computes a global shadow factor for the whole scene

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Testing status

Manual Tests: What did you do?

• Opened test project + Run graphic tests locally
• Built a player
• Checked new UI names with UX convention
• Tested UI multi-edition + Undo/Redo + Prefab overrides + Alignment in Preset
• C# and shader warnings (supress shader cache to see them)
• Checked new resources path for the reloader (in developer mode, you have a button at end of resources that check the paths)
• Other:

Automated Tests: What did you setup? (Add a screenshot or the reference image of the test please)

Yamato: (Select your branch):
https://yamato.prd.cds.internal.unity3d.com/jobs/902-Graphics

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Comments to reviewers

There's a default cloud texture named DefaultCloudLayer included in the package.