Investigate adding lightning support for OKEx

[sebastienverreault]

OKEX supports the lightning network. Currently the API does not seem to allow for creation of invoices that could be used to fund the account or withdraw from it. For deposits, a manual invoice has to be created using the Account's Web interface. Once done, it then shows up via the API getDepositAddresses, which in turn can be used to fund the account at that point. We expect that invoice to eventually be allowed to be created via API or we can look into a Selenium like control to do the invoice creation meanwhile.

Withdrawals are completely manual via the Web interface, but might be less of an issue as margin calls tend to require deposits that can't likely wait 6 on chain confirmations when a short squeeze is unrolling. Short-term options might be to Selenium-like control the transfer to Wallet created invoice.