Rework network option passing and resolution

[wachulski]

Currently network option may be:

1. network name like 'mainnet'
2. network RPC url (when starting with http)
3. #47 -> as Ganache.Provider

this complicates getSigner in config.ts

Refactor this.

Idea: Improve by introducing disciminated union types (possibly combining with infura/alchemy API keys).