Included new fields

We've added "symbol" field to these NFT endpoints responses:

/nfts/markets
/nfts/{id}
/nfts/{assets_platform}/contract/{contract_address}

Example of responses:

{
	"id": "pudgy-penguins",
  "contract_address": "0xBd3531dA5CF5857e7CfAA92426877b022e612cf8",
  "asset_platform_id": "ethereum",
  "name": "Pudgy Penguins",
  "symbol": "PPG", 👈
  ....
},