BG-11612 use FromBase58 to sign with key from path

[dannydeezy]

Motivation

https://bitgoinc.atlassian.net/browse/BG-11612

Overview Of Changes

When a user signs, we expect them to enter a key of the xprv format. However this line I'm changing is incorrect as its expecting a hex-formatted key

[cooncesean]

@dannybitgo you need eyes on this? Is this ready for review?

[dannydeezy]

@dannybitgo you need eyes on this? Is this ready for review?

yes this does need a review

[dannydeezy]

and its ready

[cooncesean]

This PR needs three things:

1. Testing steps; what are the manual steps we can take in order to test the unhappy path (basically, the issue you are fixing w/ this PR)?
2. Testing steps we can take to test the patch and the happy path.
3. Unit tests to cover the change.