fun verifyPurchase(productId: String, base64PublicKey: String, signedData: String, signature: String): Boolean
Verifies that the data was signed with the given signature, and returns the verified purchase. The data is in JSON format and signed with a private key. The data also contains the purchaseState and product ID of the purchase.
productId - the product Id used for debug validation.
base64PublicKey - the base64-encoded public key to use for verifying.
signedData - the signed JSON string (signed, not encrypted)
signature - the signature for the data, signed with the private key