> 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 [[https://github.com/bitcoin/bitcoin/pull/23595 | core#23595]]
with a minor change from [[https://github.com/bitcoin/bitcoin/pull/23413 | core#23413]] in key_io_tests (should have been included in D14768 if we got the backports in the right order)
Depends on D17516