Make certificate fields of pkcs7 data structures publicly available

[gabor-toth-arm]

Suggested enhancement

I would like to extract Subject Common Name and tbsCertificate from a mbedtls_pkcs7 object to implement UEFI variable authentication. I need these data from different certificates in the chain, so I also need to be able to iterate through the chain.
For this I need to access the listed fields:
mbedtls_pkcs7.private_signed_data.private_certs.subject
mbedtls_pkcs7.private_signed_data.private_certs.tbs
mbedtls_pkcs7.private_signed_data.private_certs.next

Currently they are only accessible as private variables.

Justification

The authentication is defined by UEFI spec, which says tbsCertificate (of the top-level issuer) and CommonName of the signing certificate's subject are need to be used.
For reference: https://uefi.org/specs/UEFI/2.10/08_Services_Runtime_Services.html?highlight=tbscertificate#using-the-efi-variable-authentication-2-descriptor