Refactor Skyplane to run on Programmable Gateways

[sarahwooders]

Tests can be run with python skyplane/test_pipeline.py.

Remaining todos:

• [x] Add multi-destination region tracking for UsageClient
• [ ] Remove all ChunkRequest references
• [ ] Convert unicast planning algorithms to gateway programs
  • [x] Direct
  • [ ] ILP
  • [ ] One-Sided Transfer
• [ ] Covert multicast planning algorithms to gateway programs
  • [x] Direct
  • [ ] MDST
  • [ ] Steiner Tree
  • [ ] ILP