Support multiple DnValue per key in DistinguishedName

[avalent]

I need to generate a certificate with a subject line containing multiple OU key-value pairs:

Subject: OU=certtype:instance, OU=compartment:compartment, OU=instance:instance

The problem is subsequent calls to DistinguishedName::push overwrite the previous value:

params.distinguished_name.push(DnType::OrganizationalUnitName, format!("certtype:{}", certtype));
params.distinguished_name.push(DnType::OrganizationalUnitName, format!("compartment:{}", compartment));
params.distinguished_name.push(DnType::OrganizationalUnitName, format!("instance:{}", instance));

This leaves me with a subject containing only the last value:

Subject: OU=instance:instance

[est31]

This needs a major change, maybe even a rename of the API so that users are aware that something changed.