Added GraphQL support for ReviewRatingStatistics

It is now possible to retrieve ReviewRatingStatistics on Products and Channels via GraphQL API.

• [GraphQL API] Added the following types to the GraphQL schema: ReviewRatingStatistics.
• [GraphQL API] Changed the Channel type:
  • Added the reviewRatingStatistics field to the Channel type.
• [GraphQL API] Changed the Product type:
  • Added the reviewRatingStatistics field to the Product type.

Introduced the following changes to the GraphQL schema (in SDL format):

extend type Channel {
  reviewRatingStatistics: ReviewRatingStatistics
}

extend type Product {
  reviewRatingStatistics: ReviewRatingStatistics
}

type ReviewRatingStatistics {
  averageRating: Float!
  highestRating: Int!
  lowestRating: Int!
  count: Long!
  ratingsDistribution: Json!
}