(PUP-1409) add authorityKeyIdentifier to client certificates

As discussed in the ticket this will add the authorityKeyIdentifier that
is not required, but recommended to add to certs signed by a CA
conforming to RFC 5280. This extension takes the subjectKeyIdentifier of
the CA certificate and adds it as keyid to authorityKeyIdentifier.  This
PR also adds a test to make sure the extension is correctly added to the
resulting certificate.
Merge branch 'issue/master/PUP1387_authorityKeyIdentifier'

* issue/master/PUP1387_authorityKeyIdentifier:
  (PUP-1409) add authorityKeyIdentifier to client certificates

Closes GH-2243