Add import_asset, export_asset, list_assets and invoice methods

[zoedberg]

After closing RGB-WG/rgb-node#117, we should add the import_asset and invoice methods

[dr-orlovsky]

I propose to have list_asset method as well, exposing runtime.list. W/o it is impossible to do a transfer after the issuance in any demo (or real-world) app

[dr-orlovsky]

BTW, invoicing does not require I9N or talking to RGB node; you can generate an invoice right in the SDK. Just construct rgb::fungible::Invoice object with it's data (ContractId, Outpoint, which may be constructed from tx outpoint data) and that's all

[zoedberg]

I think we should add also an export_asset method. @dr-orlovsky do you agree? (if yes I will add it also to RGB-WG/rgb-node#117)

[dr-orlovsky]

Yes, I think it is needed