Strongly type `pubKey` and `address`

[clangenb]

Both are just strings now, and even more confusingly, they represent the same account. It is extremely easy to mix those two up. So we should strongly type those two. I suggest using something like this:

class Pubkey {
   String value;
}

class Address {
  String value
}

Additionally, we can implement custom serializations:

// this is probably not 100% correct, but I think I can convey my idea.
String toJson() {
    self.value;
}

[clangenb]

We have already introduced and re-exported the Address type in the ew_polkadart package, we should switch to using it everywhere.