Instantiate wallet from "secret numbers" (XLS-12)

[mDuo13]

The Secret Numbers standard (XLS-12) defines an alternate format for an account seed or private key, which is used by XUMM wallet and possibly others. The Wallet class should support instantiation from an array or string of secret numbers.

[JST5000]

Here's a PR for the same task in xrpl-js which might be useful as a reference - https://github.com/XRPLF/xrpl.js/pull/1799

[JST5000]

Completed in the 2.0.0 beta release