Added Message for changes on supply channels on Stores

You can now subscribe to Messages about additions, removals, or overwrites of inventory supply Channels on Stores.

Changes:

• [API] Added StoreSupplyChannelsChanged Message.
• [GraphQL API] Added the following type to the GraphQL schema: StoreSupplyChannelsChanged.

The following changes were introduced in terms of GraphQL SDL:

type StoreSupplyChannelsChanged implements MessagePayload {
  addedSupplyChannels: [Channel!]!
  removedSupplyChannels: [Channel!]
  addedSupplyChannelsRef: [Reference!]!
  removedSupplyChannelsRef: [Reference!]!
  type: String!
}