Added support for expanded Channel and Customer Group to GraphQL query for StandalonePrice

We understood there is a demand to retrieve the full information about the Channel and the Customer Group on Standalone Prices instead of just the reference to those via the GraphQL API. Before, additional API calls were needed to retrieve the respective information, from now on just one query is sufficient for this use case.

Changes:

• [GraphQL API] Changed the StandalonePrice type:
  • Added the channel field to the StandalonePrice type.
  • Added the customerGroup field to the StandalonePrice type.

The following changes were introduced in terms of GraphQL SDL:

extend type StandalonePrice {
  channel: Channel
  customerGroup: CustomerGroup
}