bip32: add a method to create ExtendedPrivateKey from SigningKey

[fjarri]

Currently the only way to create an ExtendedPrivateKey is to generate it from a seed. I would like to use bip32 to derive child keys for DKG shares, where a signing key (or rather a share of one) has already been created by different means. Would you accept a PR that adds this functionality? Such a constructor would probably have to take an external chain code as well, since it'll have to be the same for all shares.

[tony-iqlusion]

@fjarri sure, that sounds fine to me