Add CA and ADCS Template metadata to Pkcs12

[cdelafuente-r7]

This adds the following metadata to Pkcs12:

• CA: the certificate authority that issued the certificate.
• ADCS Template: the AD CS certificate template used to issue the certificate.

The data is now serialized in the Pkcs12's data field with the following format: msf_pkcs12:<base64 cert>:<ca>:<ADCS template>.

This is a draft until we've wired up the corresponding UI changes in Pro to support consuming this metadata.

Also, this would require additional migration logic to support going from the old format to the new format, otherwise folk that have existing pkcs12 certs in their database might run into problems - but we can cross that bridge once things are wired up into Pro.

[smcintyre-r7]

Wouldn't we want something to de-base64 encode and then load the DER data into a certificate object we can use?

If that doesn't fit here because this is for presentation, then maybe at least adding another method to handle that conversion.

[cdelafuente-r7]

I believe the #openssl_pkcs12 method below does something similar. It returns a OpenSSL::PKCS12 object, from which we can extract the certificate.

So we have #pkcs12 returning the based-64 encoded Pkcs12 and #openssl_pkcs12 returning a OpenSSL::PKCS12 object.

I'm also happy to change this logic if needed.