addAccount method
      
void
addAccount({ 
    
- required String pubkey,
 - required AccountType type,
 - required EventSigner signer,
 
adds an Account
Implementation
void addAccount(
    {required String pubkey,
    required AccountType type,
    required EventSigner signer}) {
  accounts[pubkey] = Account(type: type, pubkey: pubkey, signer: signer);
}