Add CODEOWNERS file

[notmandatory]

This should match the lead maintainers mentioned in the top level BDK file here: https://github.com/bitcoindevkit

See: https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners

[sdmg15]

You mean it should match this list?

[notmandatory]

@sdmg15 yes these are the current owners, but since you're a new contributor I don't recommend this as your first issue. See: https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md#first-time-contributors