Support `LendingProtocol` (XLS-66)

[pdp2121]

Spec: https://github.com/XRPLF/XRPL-Standards/tree/master/XLS-0066-lending-protocol
rippled PR: XRPLF/rippled#5270

[realonbebeto]

@pdp2121 Is this crate by standard a port from the xrpl-py because of this "Based off of the xrpl-py library"?

I would like to handle this enhancement and from the spec this enhancement depends on and XLS-65(Draft PR) and XLS-64.

Please help confirm if XLS-64 has been implemented and if #158 (@e-desouza) needs to be merged first.