A helper function to import private key for management-external-key

A helper function to import private key for management-external-key

- Leverage keymgmt_import through EVP_PKEY_new_fromdata() to
  import "management-external-key"

- When required, use this to set SSL_CTX_use_PrivateKey

The sign_op is not implemented yet. This will error out while
signing with --management-external-key. The next commit
fixes that.

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20211214165928.30676-7-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23443.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>