Minor perf improvements and allow to get certificate information externally

[molinch]

Hey @alexalok

Would you be open to a small PR?

It improves performance when creating the ApnsClient from a certificate and allows external usage of GetCertificateInfo.

Performance changes:

• Don't allocate the array used for splitting (single reused instance)
• Avoid re-splitting
• Avoid operations when non VoIP, and use substring instead of Replace

[alexalok]

Hi @molinch,

Thank you for your improvements! LGTM, will release a new nuget version shortly.