Added option to split PDF into multiple parts and merge into one PDF (#841)

* Added option to split PDF into multiple parts and merge into one PDF

* Use the mergeDocuments method in MergeController to implement merging

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Co-authored-by: Eric <71648843+sbplat@users.noreply.github.com>

src/main/java/stirling/software/SPDF/model/api/SplitPdfBySectionsRequest.java

@@ -15,4 +15,7 @@ public class SplitPdfBySectionsRequest extends PDFFile {
 
     @Schema(description = "Number of vertical divisions for each PDF page", example = "2")
     private int verticalDivisions;
+
+    @Schema(description = "Merge the split documents into a single PDF", example = "true")
+    private boolean merge;
 }