#28697 Add environment variable for specifying location of certs file

Adds a check for the `SHOTGUN_API_CACERTS` environment variable for the location of the certificates file to use when validating SSL connections. Allows studios to use their own certificate store automatically without having to instantiate the Shotgun API instance with this parameter explicitly. This is especially useful when using distributed applications like SG Toolkit with a server that has an internal or corporate CA signed certificate.

Also updated certificates to the latest list published by Mozilla.

Author: KP