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