loginExternalSigner method
adds a new read-only Account and sets the logged pubkey
Implementation
void loginExternalSigner({required EventSigner signer}) {
final pubkey = signer.getPublicKey();
if (accounts.containsKey(pubkey)) {
throw Exception("Cannot login, pubkey already logged in");
}
addAccount(
pubkey: pubkey, type: AccountType.externalSigner, signer: signer);
_loggedPubkey = pubkey;
_notifyAuthStateChange();
}