Expose import_asset, export_asset and list_assets methods

[zoedberg]

let's add to the i9n module the methods:

• import_asset, calling the method in src/contracts/fungible/runtime.rs
• invoice, defined as here https://github.com/LNP-BP/rgb-node/blob/master/src/cli/fungible.rs#L412-L429 (Add import_asset, export_asset, list_assets and invoice methods rgb-sdk#28 (comment))
• list_assets (Add import_asset, export_asset, list_assets and invoice methods rgb-sdk#28 (comment))
• export_asset

@dr-orlovsky please correct me if I'm wrong

[dr-orlovsky]

That's right, and list method as well

[dr-orlovsky]

Seems like this issue is a blocking and a priority for v0.2 release. @zoedberg will you try to implement the methods or should I work on it myself?

[zoedberg]

Currently working on RGB-WG/rgb-sdk#24 and RGB-WG/rgb-sdk#29, but I can temporarily pause those to do this.
Actually I've already started working on this some days ago, then encountered some obstacles (while updating the issue call params on the sdk, task required to switch to latest rgb-node release) and so I paused the job. If you want to do this task, I can push my intermediate results so you already have something to start from

[dr-orlovsky]

I can do the stuff quick, but I thought you are interested in trying to play with the Node code, not just SDK around it, and this may be a good first issue. Your WIP PR will be in any way useful

[zoedberg]

I am interested, but currently a bit overloaded by tasks. I will give it another try tomorrow, and if not succeed quickly, I'll open a draft PR

[dr-orlovsky]

Yeah, feel free to postpone other tasks, they are not that critical to do the release