How to handle the SS58-prefix in the dart API.

[clangenb]

Currently, we have some RPCs that return AccountIds. Substrate encodes them by default with the generic SS58-prefix 42. This is transformed into the node-specific SS58-address format by the client side, e.g., polkadot-js/api usually.

In our dart api, we don't have such a logic. Hence, this leads to inconsistency in the address format, when compared to the values returned by the polkadot-js api.

#568 hardcodes the prefixes to 42 for all networks, to fix inconsistencies with the js-api, which would use the network-specific prefix. This fix might be acceptable if we assume that the users will never use their account with polkadot-js/apps or with subscan. But is this a valid assumption?

We might want to internally the prefix 42 always, but we show the network specific one to the user.