Support Union type

[jiqiang90]

Add support for GraphQL Union Types. Unions and interfaces are abstract GraphQL types that enable a schema field to return one of multiple object types. e.g. union Media = Book | Movie

Acceptance Criteria

• You must be able to define union types using the union type e.g. union Media = Book | Movie
• You can use them in entities like

union Media = Book | Movie 

type Content @entity {
  id: ID! #id is a required field
  media: Media!
  content: String!
}

[2075]

hey there, any update on this 'nice to have' ? :)

[stwiname]

This is largely dependent on https://github.com/subquery/subql/issues/1982