Add new `/sign-multichain` route

[esaminu]

Description

• [ ] Render only when receiving a window.postmessage containing transaction + key derivation path in borsh
• [ ] When window.postmessage is received compare the key derivation path with the origin and transaction chain and accountId
  • if accountId is wrong dismiss with error
  • if asset is not supported dismiss with error
  • if domain is wrong, alert UI
  • If domain is correct succeed without UI
  • If domain is not provided, show confirmation
• [ ] Deserialize transaction and determine receiver, chain and amount. Should be able to deserialize
  • BNB on BNB
  • ETH on ETH
  • BTC on BTC
• [ ] Ideally fetch gas and set it
• [ ] Sign the transaction and send back to demo app by postMessage

Instructions on snippet: https://gist.github.com/esaminu/8ecfd8524a3404d34135acde9c7cbb8b