[MOVEONLY] Move ParseHDKeypath to utilstrencodings

Summary:
This is a partial backport of Core PR13723 : https://github.com/bitcoin/bitcoin/pull/13723/commits/3b01efa0d1bf3d23d1b7b7e518849f1fc26314f9

Depends on D4486

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4488