Add Support for Prysm's Sapphire Testnet

[sorpaas]

This adds basic support for syncing with Prysm's Sapphire testnet. To test this out, you will need to run a local Prysm node:

docker run --rm -v $HOME/workspace/prysm-data:/data -p 4000:4000 -p 13000:13000 --name beacon-node gcr.io/prysmaticlabs/prysm/beacon-chain:latest --datadir=/data

Run Shasper with --chain sapphire flag and specify the prysm node as one of the local node to connect to.

RUST_BACKTRACE=1 RUST_LOG=info cargo run --release --bin shasper-blockchain -- --chain sapphire --port 3001 --libp2p-nodes /ip4/127.0.0.1/tcp/13000