Add token after fiat on ramp order

On v10.10.0 we [enable the purchase of stablecoins via the on ramp feature](https://github.com/MetaMask/metamask-extension/pull/13398) on BSC, Polygon, Avanlanche, Fantom, Celo and Ethereum Mainnet.
But if a user purchase stablecoins and doesn't have token detection on or is in a network where we don't support token detection, the stablecoin won't appear in the users' wallet and make them confused.
To avoid this confusion, **we should automatically add the stablecoin to the users' wallet after a successful on ramp order.** This is similar to what we currently do for swaps.

For reference:
[Slack thread](https://consensys.slack.com/archives/G018FR294E9/p1645112468008049)
[Mobile PR](https://github.com/MetaMask/metamask-mobile/pull/3797/commits/9a1189960e7ac31baacc088f307065d6bb493118)

Start by comparing the mobile PR linked in the issue with the extension's existing swap behavior for adding tokens. Trace the successful on-ramp order flow and identify where the purchased stablecoin can be added to the wallet; done means the stablecoin appears after a successful order even when token detection is disabled or unsupported.

Written by the indexing model from the issue text.