Enable Subscription SNS topic for SaaS Contract pricing model

[chotafakir]

Is your feature request related to a problem? Please describe.
The Subscription SNS topic was not available for SaaS Contract model. This topic covers the subscription status of a marketplace customer. It is now available for all SaaS pricing models.

Describe the solution you'd like
Setup the logic to subscribe to Subscription SNS topic for SaaS contract product and send the message to the Seller via the Support SNS topic.

Additional context
https://docs.aws.amazon.com/marketplace/latest/userguide/saas-notification.html#saas-sns-subscription-message-body