Add tokens to MetaMask using wallet watchAsset bug

[zhaonancy]

MetaMask didn't add cUSD, cEUR, cREAL automatically when switching to a new network. Found when investigating #146

When calling addTokensToMetamask which uses wallet_watchAsset (https://github.com/celo-org/use-contractkit/blob/master/packages/use-contractkit/src/utils/metamask.ts#L137), a popup prompts the user separately for each token every time, even if the token has already been added.

The wallet_watchAsset request return type is documented as a boolean for if the token is added, but actually returns an object immediately with status as pending, found when @nicolasbrugneaux and I tried storing the boolean in local storage to know whether to call addTokensToMetamask in order to avoid the recurring popup.

Previous MetaMask Integration PR: #80

Docs for wallet_watchAsset:

• https://docs.metamask.io/guide/rpc-api.html#wallet-watchasset
• https://docs.metamask.io/guide/registering-your-token.html#example
• https://github.com/ethereum/EIPs/blob/master/EIPS/eip-747.md

MetaMask Issues related to wallet_watchAsset (not working in Firefox, immediate return in Chrome):

• Method "wallet_watchAsset" does't work correctly MetaMask/metamask-extension#12416
• Function wallet_watchAsset not working as expected in Firefox. MetaMask/metamask-extension#11377
• Method to get assets that have been already listed in MetaMask MetaMask/metamask-extension#7360

[nicolasbrugneaux]

With @zhaonancy we double checked the state of that issue. Metamask has the fixed merged in, and released a build with it. We tried it and it seems to fix our issues.

However tha specifc fix isn't part of released extension on the chrome/firefox store yet.
PR in question: MetaMask/metamask-extension#14545