Add support to multiple accounts in the same wallet (NEP-6)

[lock9]

Summary or problem description
Wallets can have multiple accounts. This issue is for including new accounts to existing wallets and creating new wallets with multiple accounts

Additional contextual information

• A NEP-6 wallet is a collection of accounts, meaning that it can contain one or multiple NEP-2 accounts.
  • The wallet should be already implemented in Wallet creation using NEP-6 #59. This issue is to include accounts to the wallet

AddAccount(acc: Account)
RemoveAccount(address: str)