[SDK-926] feat: added optional parameter to filter meshes

[HarrisonHough]

SDK-926

Description

• This improves the get mesh renderer function to optionally filter for null skin meshes. This is useful when you are using the AvatarTemplate prefab which has all possible skinnedmeshrenders (for mesh transfer purposes). Old functionality would return the first head mesh out of these three "Renderer_Head", "Renderer_Avatar", "Renderer_Head_Custom" regardless of whether they were empty or not which causes the eye blinking not to work.
• I also added an extra UpdateHeadMesh() method which would be useful if you want to find and set the head mesh from code, the usecase would be if you are loading avatars with different configurations EG Texture atlas and non texture atlassed using the mesh transfer technique

How to Test

• 1 way to test would be to simply drag the RPM_Template_Avatar_XR prefab into the scene and add the EyeAnimationHandler and run to see if the blinking works.

Checklist

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