Add dog_v2/ directory and build_dog script to locomotion/walkers/assets/.

- Preparation for improved dog model.
- Added dog_base.xml with all the defaults.
- Moved floor and tennis ball to scene.xml, removed target.

PiperOrigin-RevId: 476119984
Change-Id: I8c5e836fe61956872973528659137fe74045badc

Author: yuvaltassa