provide offline methods

[zoedberg]

The current implementation of Runtime requires a valid electrum_url in order to be loaded. This means that all RGB methods require to be online, even methods that do not have this requirement because they actually use only data saved on disk (e.g. iface_by_name and contract_iface to get asset metadata). I propose to make all methods that do not require a blockchain resolver available even when offline.

I think there are many ways to implement this. One solution could be: making the electrum_url parameter of Runtime::load an optional one, then of course make also the resolver Runtime's' field optional and check for it's presence only on methods that actually need it, returning an error if the Runtime has been instantiated without an electrum URL.

@dr-orlovsky what do you think?

[dr-orlovsky]

Good point, will work on that!