Feature request: Add option to add Avatar upload field to PMPro Checkout

[ipokkel]

Is your feature request related to a problem? Please describe.
PMPro Members can only add their custom avatar on their profile update page after becoming a user and not during registration

Describe the solution you'd like
An option for Basic User Avatars to include an avatar file field on PMPro checkout If PMPro core is active so that users of the BUA plugin can optionally include the upload field for their member checkout.

Describe alternatives you've considered
While using BUA v 1.0.7, implemented similar functionality by creating a user field in PMPro settings and setting the custom avatar for BUA with customization code - https://gist.github.com/ipokkel/7a026f6002232a56e1dc448ff58da6fd