[Terrain] Refactor so tmat use Texture instead of string

[PolEpie]

Pull Request

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Summary

Before you could only put path to a texture and it fetched it, but no operation were possible unlike what Decals have with builtin Texture type

Motivation & Context

I wanted a way to add roughness easily #11510 but then wheatley point out that adding this feature would be better than just a roughness slider

Implementation Details

So i did a huge refactor cause we gotta do a json upgrade, but basically i changed every slot for Texture type, a upgrader and i did change how we generate the texture then, so i unify the template (thx jean claude) and create Bitmap.PackChannel so we can build the baked texture easily

Screenshots / Videos (if applicable)

Now :

Checklist

• Code follows existing style and conventions
• No unnecessary formatting or unrelated changes
• Public APIs are documented (if applicable)
• Unit tests added where applicable and all passing
• I’m okay with this PR being rejected or requested to change 🙂