Populate `ChainSpec` based on information stored in `genesis.json` file

[mpaulucci]

When starting a localnet, we get the chain spec information in the genesis.json file. See: https://github.com/lambdaclass/ethereum-package/pull/1/files#diff-79bf8330a2e72edba847d9ca9de634c4339dc13afb11636b13e0a4775119dcbbR200

This will look like network=/network-configs/genesis.json.

A genesis.json file looks like this: genesis.json

We want to read that file and populate ChainSpec struct.
Also we need to parse the account info for the initial accounts.
More information in needed related to how ChainSpec will look like.