Add support for nesting animation groups

[cabmeurer]

Summary

This is to add basic support for nested animation groups i.e adding method addAnimationGroup(...) to AnimationGroup that mirrors addAnimation(...) per issue #25

Changes:

• Added the addAnimationGroup method, allowing nested AnimationGroup instances.
• View Controller Example:
  • NestedAnimationGroupViewController Provides a simple sequential nested animation example.
• Tests:
  • AnimationGroupTests: Added testAddAnimationGroup to verify the functionality of nested animation groups.
  • SnapshotTestingFrameImageTests: Added testNestedAnimationGroupSnapshot to capture the visual representation of nested animation groups.
  • SnapshotTestingAPNGImageTests: Added testNestedAnimationGroupSnapshot to verify the animated representation of nested animation groups.

Notes

Cherry-picked snapshot test updates from branch entin/iOS-15-snapshots to ensure consistency and inclusion of the latest testing improvements.