Add configurable network ID and include it in transaction data binaries when signing

[d-velev]

The network ID is appended to the transaction data that is being signed, meaning that transactions made in one network aren't going to be valid in another. It should be "ae_mainnet" by default

[Artur64]

@d-velev Could you provide a link to get more information about this?

[d-velev]

@Artur64 I'm not sure where their signing is happening but this is how they transform their transaction binaries https://github.com/aeternity/epoch/blob/master/apps/aecore/src/aec_governance.erl#L217. So we must sign "ae_mainnet" <> serialized_tx and verify the same way