Add configuration option for using the backup port on Asp.Net Core

[prichter98]

I have encountered a problem trying to use the ApsNet.Core integration package. I need to use the backup port but there is no way to configure the ApnsService to create clients using the backup port.

If the service could be configured with an ApnsServiceOptions setting, analogous to DisableServerCertificateValidation, the problem would be neatly solved.

[legoesbenr]

This seems lige the same need to configuring the apns service as I have in #136.