Retrieves the allowance of a specified ERC20 token for a given owner and spender.
GET //erc20/{account}/allowance HTTP/1.1
Host: beta.usemoon.ai
Authorization: text
Accept: */*
A promise that resolves to an ERC20APIResponse containing the allowance as a string.
{
"success": true,
"message": "text",
"data": "text"
}