[SDK-699] Add shader property in avatar config

[rYuuk]

SDK-699

Description

• Add custom shader property in avatar config
• Removed avatar shadow from cac

How to Test

• Load avatar with Avatar config standard shader set in avatar config in settings

Checklist

• Tests written or updated for the changes.
• Documentation is updated.
• Changelog is updated.

[HarrisonHough]

Overall the code looks good.
1 improvement idea is that we could look at showing only shader properties (or rather texture channels) that have been enabled?

EG in this case only BaseColor would be visible under the Shader section

Also as mentioned in slack but maybe it makes sense to explore the gltfast instantiator approach for assigning materials/shaders to prevent the "double" assigning. Or at least test if doing this has any benefit performance wise