[GRDSubscriberCredential] add class properties

[tzeejay]

Please add the follow class properties

• jwt type string
• subscriptionType type string
• subscriptionTypePretty type string
• subscriptionExpirationDateUnix type Long
• subscriptionExpirationDate type Date
• tokenExpirationDateUnix type Long
• tokenExpirationDate type Date

The majority of this is hidden in SubscriberCredentialsJSON. Please remove the class and move the JSON annotations over to GRDSubscriberCredential. The properties subscriptionExpirationDate & tokenExpirationDate should have no JSON annotation as those will be computed from the Unix timestamps during parsing