Add 'sethdseed' RPC to initialize or replace HD seed and test

Summary:
The first commit adds a new command sethdseed which allows you to either set or generate a new HD seed to be used. The second commit adds a test for this.

Partial (2/4) backport of Core PR12560
https://github.com/bitcoin/bitcoin/pull/12560/commits/b5ba01a18707b1d50c4d073bd40e1e92d78d04c1
https://github.com/bitcoin/bitcoin/pull/12560/commits/2bcf2b52aea07c778b439e6d6931bb4a1733304

Depends on D4201

Test Plan:

make check
test_runner.py

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4202