NFT API: Added royalty payers endpoint

[Dec 13, 2023]

1. We added a new endpoint Get royalty payers by contract, you can retrieve the all the payers of a collection, and get each payer's royalty info including royalty_expense, outstanding_royalty_expense, average_royalty_fee_rate_per_sale
2. For the existing endpoint Get holders by contract, we added the parameteroutstanding_royalty_expensein the response field.