We have a Java Application and we are invoking Appsync/GraphQL for search Service. Appsync searches different systems based on the search criteria passed, consolidate the results(for matching records) and sends it back to our application. The response is a common JSON schema but fields varies based on the system the Search criteria has matched. There is a query parameter which specifically says which system to search and I know which fields I need to query and get back in the response.
I need suggestions on what is the best design pattern I can use in my java application to build fields dynamically that needs to be in the schema(as I know the target system and fields available at the system). I am planning to use a properties file and hold placeholders for each system and its corresponding fields(that I want in response) and pass this while building the query.

For example GraphQL schema is as below

BooksSchema {
       bookDetails {
        bookName
        bookEdition
        bookcolor
        }
        authorDetails {
          authorName
          authorAge
          authorPublication
        }
        bookPrice {
          totalCost
          costExclVat
          vatPercentage
          costInOtherCountry
      discount  
        }
       }
      }
}

Query fields should be like below when libraryName is High Street London as I know bookPrice is not available at this library

query MyQuery {
  searchBooks(bookSearchRules: {authorName: "Kipling", bookName: "Jungle Book", libraryName: "High Street London"}) {
    ... on BooksSchema {
       bookDetails {
        bookName
        bookEdition
        bookcolor
        }
        authorDetails {
          authorName
          authorAge
          authorPublication
        }
       }
      }
}

Query fields should be like below when libraryName is High Street Reading as I know authorDetails is not available at this library

query MyQuery {
  searchBooks(bookSearchRules: {authorName: "Kipling", bookName: "Jungle Book", libraryName: "High Street Reading"}) {
    ... on BooksSchema {
       bookDetails {
        bookName
        bookEdition
        bookcolor
        }
        bookPrice {
          totalCost
          costExclVat
          vatPercentage
          costInOtherCountry
          discount  
        }
       }
      }
}