feat(cardano): simplify Asset type

[carbolymer]

Since currently both of the variants of quantity can store negative values, there is no point of having them both if they serve the same purpose.

There is a problem though: BigInt can store negative values, which do not make sense in case of assets in transaction outputs. One way to approach this would be to keep sum type quantity but use two different types: BigInt and some new type representing only positive values for example:

message BigUInt {
  oneof big_uint {
    uint64 int = 1;
    bytes big_u_int = 2;
  }
}

The question is if the introduced complexity justifies the gain in type safety.