IK Retarget editor's asset browser doesn't list all asset types

Certain asset types that are displayed in the Retarget Animation window are not displayed within the asset browser in the IK Retarget editor

1. Open the Game Animation Sample
2. Find any animation, r-click and select Retarget Animations
3. In the Retarget Animation window, note the type of assets that are displayed
   1. Anim bps are listed
4. Now open a IK Retargeter asset (like RTG_UEFN_to_Echo)
5. In the content browser within the retarget editor, note the asset types displayed
   1. Anim bps are not listed