Not open-source, and real rough at the moment but it's a combination of these three things + some custom systems to determine which way to apply gravity when you touch a surface.
- https://weaverdev.io/projects/bonehead-procedural-animation/ - Basic IK Rig
- https://www.youtube.com/watch?v=qdskE8PJy6Q - Third-person character controller
- https://www.omabuarts.com/product/chameleon-quirky-series/ - Model
Sorry, I know that's not super helpful unless you've got the time / knowledge to develop it from scratch, but can't share it as it's not entirely my work.