Reduce duplication in opening and reading the pem file for cert content in ClientCertificateCredential

[ahsonkhan]

          `ReadPemCertificate()` already does open the file and read its contents. Can you make it so that x5c gets read as part of that process, instead of opening the file for the second time and reading its contents again?

Originally posted by @antkmsft in #5988 (comment)

[github-actions]

Thanks for the feedback! We are routing this to the appropriate team for follow-up. cc @Jinming-Hu @microzchang @vinjiang.