chore(account-lib): export polkadot interface
chore(account-lib): export polkadot interface

In order to use the type def outside of account-lib, we will
need to export the type like eth or celo do.

Ticket: STLX-8335

BitGo/BitGoJS,BitGo/BitGoJS,BitGo/BitGoJS,BitGo/BitGoJS