ADDR-1689 Add exclude source object option to variants project

[kirstenpilla]

• Any groups that use the texture or prefab variant schema, must also have a bundled asset group schema with their "IncludeInBuild" flag enabled. If disabled, neither the original source group or its variant groups will be included in the build.

• Added option "IncludeSourceTextureInBuild" to the texture variant schema. Disabled by default. If the option is disabled, the custom build script will temporarily disable the bundled asset group schema "IncludeInBuild" flag to exclude the original source texture group from the build.

• Added option "IncludeSourcePrefabInBuild" to the prefab variant schema. Disabled by default, but since the sample project expects to load the source prefab it is enabled in the project. If the option is disabled, the custom build script will temporarily disable the bundled asset group schema "IncludeInBuild" flag to exclude the original source prefab group from the build.

[AndyMUnity]

Could include source in build also be extended to the prefab variants?

[kirstenpilla]

Could include source in build also be extended to the prefab variants?

It's possible. I noticed that we still load the original prefab in the sample scene. I guess I can add the option, but instead make it enabled for the sample.