Add ERC7739 and ERC7739Utils (#5664)

contracts/utils/cryptography/AbstractSigner.sol

@@ -0,0 +1,22 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.20;
+
+/**
+ * @dev Abstract contract for signature validation.
+ *
+ * Developers must implement {_rawSignatureValidation} and use it as the lowest-level signature validation mechanism.
+ *
+ * @custom:stateless
+ */
+abstract contract AbstractSigner {
+    /**
+     * @dev Signature validation algorithm.
+     *
+     * WARNING: Implementing a signature validation algorithm is a security-sensitive operation as it involves
+     * cryptographic verification. It is important to review and test thoroughly before deployment. Consider
+     * using one of the signature verification libraries (xref:api:utils#ECDSA[ECDSA], xref:api:utils#P256[P256]
+     * or xref:api:utils#RSA[RSA]).
+     */
+    function _rawSignatureValidation(bytes32 hash, bytes calldata signature) internal view virtual returns (bool);
+}