Support image param in wallet_watchAsset

[yrliou]

Currently we do not support providing the image URL for custom tokens via wallet_watchAsset because we don't support getting a remote resource yet.

Test Plan:

1. Visit https://www.coingecko.com/en/coins/railgun and click on the fox icon to add suggest token

2. Prompt should show up for adding suggest token (will be asking to unlock first if wallet is locked), make sure the icon is displayed as expected here.
   

3. Click to add the token and check the token is added to user visible asset list and make sure the icon is displayed as expected here.
   

[srirambv]

Brave	1.36.90 Chromium: 98.0.4758.87 (Official Build) beta (64-bit)
Revision	e4cd00f135fb4d8edc64c8aa6ecbe7cc79ebb3b2-refs/branch-heads/4758@{#1002}
OS	☑️ Linux	☑️ Windows 11 Version Dev (Build 22523.1000)	☑️ macOS Version 12.0.1 (Build 21C52)
Verified test plan from issue description Verified Add token shows icon for the coin that is being added Verified custom asset added shows the icon on visible asset list
	20000-Linux.mp4	20000-Windows.mov	20000-macOS.mov