Add a filter to allow the consumer to receive customised information from the server

I have a need to receive extra information back from the server. In particular, I want to receive information that says "updates entitlement soon expires" or "updates entitlement already expired".

Currently, the functions in the class PluginUpdate_1_3 strip out anything not in the private $fields.

This change adds a filter to allow the consumer to retain any extra fields they wish.