Use iconUrls from add-ethereum-chain to populate network icons

[kevinghim]

Background

We don't currently have a decentralized way to make changes to network metadata such as network name and icons. We frequently get requests to add or change icons -- we should consider making the chain logo part of the add-ethereum-chain api, and also the custom network form. https://github.com/ethereum-lists/chains already allowing PRs to specify icons for chains added to the DB, so whenever a network has a request to update metadata, we'll point them to make changes there.

Acceptance criteria

• store iconUrls passed in from the add-ethereum-chain api are stored as part of the network configuration
• wallet should save the image on its first request
• cache it locally (to reduce IP trackability)
• create a field on the custom network form for users to
• provide a UI to give users control when there are multiple RPCs with the same chainId [optional]

[marianoangra]

E para Polygon?