Add Strings.toString for signed integers (#3773)

Co-authored-by: Hadrien Croubois <hadrien.croubois@gmail.com>
Co-authored-by: Gbolahan <89295500+GbolahanAnon@users.noreply.github.com>
Co-authored-by: Francisco Giordano <frangio.1@gmail.com>

contracts/mocks/StringsMock.sol

@@ -9,6 +9,10 @@ contract StringsMock {
         return Strings.toString(value);
     }
 
+    function toString(int256 value) public pure returns (string memory) {
+        return Strings.toString(value);
+    }
+
     function toHexString(uint256 value) public pure returns (string memory) {
         return Strings.toHexString(value);
     }