Allow to add custom blockchain networks to wallet

[spylogsster]

At the moment we only have the ability to add EVM networks. In the process of integrating Solana, Filecoin and others, we need to refactor and allow to use arbitrary list of networks. Preliminary structure from Slack discussion:

struct BlockchainNetworks {
 EthereumNetwork[] evm_networks,
 FilecoinNetwork[] file_networks,
 SolanaNetwork[] sol_networks,
}

and to share the common network properties, we can probably do

struct EthereumNetwork {
   NetworkBase base;  // naming TBD
   // specific properties for EVM
   bool is_eip1559;
}