Design/finalize custom asset API

[danfinlay]

The current plugin beta has a custom asset management api, which allows plugins to list their assets in the main token list.

I say asset instead of tokens, because these don't need to be tokens, they can be anything a plugin chooses to render. So far we've seen a few of these built!:

• Layer 2 assets
• Other blockchain tokens
• Collectibles
• Incoming messages
• Subscriptions
• Allowances

The current implementation allows just a few things to be provided here:

• an asset "symbol" (short description)
• an asset icon
• a longer name
• a numeric value/quantity
• a link to visit on click. Clicking may result in showing a custom view in-page: https://github.com/MetaMask/metamask-snaps-beta/issues/103

There are also some security critical features of these.

• It should be easy to distinguish the (plugin?) source of a given asset (label the provenance).

[rekmarks]

Complex feature that will be tracked elsewhere.