Initial support for OpenSSL x509 TRUSTED CERTIFICATE.

[adiroiban]

Scope

Fixes: #28

This is just the initial implementation for a trusted certificate.

No extra handling is done to compare a normal certificate with its associated trusted certificate

The OpenSSL is included in the name of the class as this is an OpenSSL only feature. See https://github.com/hynek/pem/issues/28#issuecomment-804096684

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[adiroiban]

This is ready for review :) I can't modify the PR metadata.

[hynek]

Thanks Adi!