[SDK-600] Feature/avatar template element

[HarrisonHough]

SDK-600

Description

• This PR includes a TemplateAvatar element that can be used to display avatar template icons and fire an event with the required data. It also includes a ButtonElement class which should be usable in other cases.
• it has I realize that we already have Asset and Avatar button classes in the sample but I wanted to keep this separate so as not to break the sample but also I think these simpler elements might be more re-usable. We can always update later as needed.

How to Test

• easiest way to test is to checkout the feature/template-avatar-element branch in Unity-SDK repo and run the test scene in the TEMP folder. You should be able to:

1. run the scene
2. see the template buttons load
3. click a button
4. see selected icon
5. check console for print out on avatar ID and gender

Checklist

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

[rYuuk]

🟡 Nice work, I nit-picked a bit😅. Please let me know if the concerns are valid or not.

[rYuuk]

🟢 Thanks for making the changes.