Add `qxListAssets` or similar to list all assets

Hi,

playing around I noticed that to get Qx bid/asks for an asset one needs to specify the asset name and issuer. That's because apparently it's possible to have multiple assets with the same name (CfB).

While for SC it's easy as the issuer is always the all-A (AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAFXIB), getting the issuer of an asset is more complicated (*). Furthermore, with more and more Tokens being created it will be hard to keep track of which assets exist, and most probably we don't want UI/tool developers to maintain an hard-coded list of assets and their issuers.

It would be great therefore to have a way of asking nodes for a list of all existing assets, SC and tokens. Opening this issue as of CfB advice.

I haven't found where this data can be found in memory yet, but judging from the other Qx calls the implementation could be something like:

struct Assets_input
{
    uint64 offset;
};

struct Assets_output
{
    struct Asset
    {
        id issuer;
        uint64 assetName;
     };

     array<Asset, 256> assets;
};

struct Assets_locals
{
    // ...
};

PUBLIC_FUNCTION_WITH_LOCALS(Assets)
   // ...
_

This assuming is a Qx function/responsibility. It could also be just a core network message. Especially dubious after readong #102 ... Again, I'm not sure where this info is in memory yet, I'll investigate more and if able I may try and provide a PR.

(*) I haven't found a way to do it yet

qubic / core

Add `qxListAssets` or similar to list all assets #109