Extracts the private key bytes from the provided encoding.
This will decode data conforming to the current spec at
https://tools.ietf.org/html/draft-ietf-curdle-pkix-04
or as inferred from the old spec at
https://tools.ietf.org/html/draft-josefsson-pkix-eddsa-04.
Contrary to draft-ietf-curdle-pkix-04, it WILL accept a parameter value
of NULL, as it is required for interoperability with the default Java
keystore. Other implementations MUST NOT copy this behaviour from here
unless they also need to read keys from the default Java keystore.
This is really dumb for now. It does not use a general-purpose ASN.1 decoder.
See also getEncoded().