REST API: Migrate Coupons endpoints

[itsmeichigo]

As part of the REST API project, we want to migrate Coupons endpoints to use application password for authentication.

Subtasks:

• Update CouponListMapper and CouponMapper to parse contents both with and without the data envelope. - [REST API] Update Coupon mappers to handle json from REST API #8466
• Update CouponReportListMapper to parse contents both with and without the data envelope. - [REST API] Update Coupon report mapper to handle JSON from REST API #8476
• Use JSON encoding for put and post methods in RESTRequest - [REST API] Use JSON encoding for RESTRequest when needed. #8500
• Update CouponRemotes to use the new request type instead of JetpackRequest. - [REST API] Migrate Coupons Jetpack tunnel endpoints to REST API #8501