Enable "Vendor Metered Tagging" feature as part of SaaS CFT

[shentao12]

Is your feature request related to a problem? Please describe.

The problem: customers does not have a clear visibility into their cost usage from SaaS provider if there is no tagging around the usage. Customers will only receive one bill that includes all the activities associated with the SaaS subscription.

Describe the solution you'd like

Enabling Vendor-metered tagging feature in the SaaS CFT solution would allow vendors to tag the usage that relevant to the customers, so customers can better understand their resource usage. The solution consist of adding the extra tagging field that SaaS vendors can supply, and support metering payload up to 1MB, because it is a new requirement after enabling tagging field.

Additional context

Documentation Link: https://docs.aws.amazon.com/marketplacemetering/latest/APIReference/API_BatchMeterUsage.html

This feature is especially important for SaaS product as we have limited information about SaaS user account. This will benefit both vendors and the customers for the long term, below are few value prepositions:

1. Customers will be able to understand the breadth of the value the seller provide for their business units.
2. This will directly reduce manual work for customers in terms of reconcile costs.
3. Increase revenue for ISVs, because better tagging will help customers increase usage if they can understand and analyze allocation usage in detail.