Add selection for PDF/A output format (#1095)

* Create PdfToPdfARequest.java

* Change class, add output format

* Add input field for output format

* Change output format selection order

src/main/java/stirling/software/SPDF/model/api/converters/PdfToPdfARequest.java

@@ -0,0 +1,17 @@
+package stirling.software.SPDF.model.api.converters;
+
+import io.swagger.v3.oas.annotations.media.Schema;
+
+import lombok.Data;
+import lombok.EqualsAndHashCode;
+import stirling.software.SPDF.model.api.PDFFile;
+
+@Data
+@EqualsAndHashCode(callSuper = true)
+public class PdfToPdfARequest extends PDFFile {
+
+    @Schema(
+            description = "The output PDF/A type",
+            allowableValues = {"pdfa", "pdfa-1"})
+    private String outputFormat;
+}