Added the 'key' field to InventoryEntry

InventoryEntry now supports the key field. You can now retrieve, update, and delete InventoryEntry by user-defined key.

Changes:

• [API] Added key field to InventoryEntry and InventoryEntryDraft.
• [API] Added Get, Update, and Delete by Key methods to InventoryEntry.
• [API] Added Set Key update action to InventoryEntry.
• [GraphQL API] Changed the Query type:
  • Query.inventoryEntry(id) type changed from String! to String
  • Query.inventoryEntry(id) description is changed
  • Argument key was added to Query.inventoryEntry field
• [GraphQL API] Changed the Mutation type:
  • Mutation.updateInventoryEntry(id) type changed from String! to String
  • Mutation.updateInventoryEntry(id) description is changed
  • Argument key was added to Mutation.updateInventoryEntry field
  • Mutation.deleteInventoryEntry(id) type changed from String! to String
  • Mutation.deleteInventoryEntry(id) description is changed
  • Argument key was added to Mutation.deleteInventoryEntry field