chore: move alpha_texture from PBRMaterial to UnlitMaterial (#223)

Added new property alphaTexture to UnlitMaterial and added @deprecated notation to PbrMaterial's alpha_texture

proto/decentraland/sdk/components/material.proto

@@ -22,14 +22,15 @@ message PBMaterial {
     optional float alpha_test = 2; // default = 0.5. range value: from 0 to 1
     optional bool cast_shadows = 3; // default =  true
     optional decentraland.common.Color4 diffuse_color = 4; // default = white;
+    optional decentraland.common.TextureUnion alpha_texture = 5; // default = null
   }
 
   message PbrMaterial {
     optional decentraland.common.TextureUnion texture = 1; // default = null
 
     optional float alpha_test = 2; // default = 0.5. range value: from 0 to 1
     optional bool cast_shadows = 3; // default =  true
-
+    // @deprecated Alpha textures are no longer supported on PBRMaterial and UnlitMaterial.alphaTexture should be used instead.
     optional decentraland.common.TextureUnion alpha_texture = 4; // default = null
     optional decentraland.common.TextureUnion emissive_texture = 5; // default = null
     optional decentraland.common.TextureUnion bump_texture = 6; // default = null