FR: add asset category tags, allow someone from product to manually add icons and add/remove such tags

Some assets registered on the asset registry are more relevant than others for end users. In the wallets UI and in the explorer [assets section](https://blockstream.info/liquid/assets) we already have several test issuances clogging the view with unrelevant asssets.

We need a way to flag a subset of the currently registered assets, as "featured_assets", so that they can be displayed in foreground on GUI clients.

Going forward, to scale a bit the operations of flagging such assets in the registry as more issuers will want their assets to be featured, we'd need someone from product to be able to:
1. add/remove a new registered asset to the featured assets list
2. add/remove/change icons for such new assets (virtually all these assets will have icons)

Start with the asset registry and the linked explorer assets section, then determine how registered assets are represented for the wallets and other GUI clients. Done means a product user can add or remove featured assets and add, remove, or change icons for those assets.

Written by the indexing model from the issue text.