feat: add java keystore certificate option for pdf signing

src/main/java/stirling/software/SPDF/model/api/security/SignPDFWithCertRequest.java

@@ -14,7 +14,7 @@ public class SignPDFWithCertRequest extends PDFFile {
 
     @Schema(
             description = "The type of the digital certificate",
-            allowableValues = {"PKCS12", "PEM"})
+            allowableValues = {"PEM", "PKCS12", "JKS"})
     private String certType;
 
     @Schema(
@@ -28,6 +28,9 @@ public class SignPDFWithCertRequest extends PDFFile {
     @Schema(description = "The PKCS12 keystore file (required for PKCS12 type certificates)")
     private MultipartFile p12File;
 
+    @Schema(description = "The JKS keystore file (Java Key Store)")
+    private MultipartFile jksFile;
+
     @Schema(description = "The password for the keystore or the private key")
     private String password;