go/beacon: Add a PVSS based beacon backend
This commit adds a PVSS-based beacon backend that hopefully is a significant improvement over the existing beacon, in that it at least attempts to be secure.
Note that certain nicer-to-have features/test cases not yet implemented:
- Checkpoint/restore of node beacon state needs testing.
- Slashing based on non-participation needs fine-grained protocol phase accounting to be secure.
- Performance with more than a handful of participants is attrocious, where more than a handful is currently approximately 20.