util: Add ParseHex<std::byte>() helper

Summary:

test: Add test for embedded null in hex string

util: Add ParseHex<std::byte>() helper

refactor: Use Span of std::byte in CExtKey::SetSeed

This is a backport of core#23595

with a minor change from core#23413 in key_io_tests (should have been included in D14768 if we got the backports in the right order)

Depends on D17156

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, roqqit

Reviewed By: roqqit

Differential Revision: https://reviews.bitcoinabc.org/D17157