Handle native token for notifications

[anxolin]

When converting events to notifications, we try to load the symbol and decimals from the token to format a new notification message.

See:
https://github.com/cowprotocol/bff/blob/main/apps/notification-producer/src/producers/trade/fromTradeToNotification.ts#L39

The native token doesn't have those.

We need to check for 0xeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee and return the native currency and decimas for each network.