Default network config

[Dancia]

To connect to data layer you have to supply NATS_URL. What if instead of each time supplying NATS_URL there was a config with default network settings. See https://pkg.go.dev/github.com/oasisprotocol/oasis-sdk/client-sdk/[email protected]/config#pkg-variables as an inspirational example. We could default to testnet with default brokers nats urls so users (99% of cases). As a result instead of knowing to find URLs here you would just get them defaulted.