pg_dump: Add const decorations

Add const decorations to the *info arguments of the dump* functions,
to clarify that they don't modify that argument.  Many other nearby
functions modify their arguments, so this can help clarify these
different APIs a bit.

Discussion: https://www.postgresql.org/message-id/flat/012d3030-9a2c-99a1-ed2d-988978b5632f%40enterprisedb.com

Author: petere

src/bin/pg_dump/pg_backup.h

@@ -248,7 +248,7 @@ typedef int DumpId;
  * Function pointer prototypes for assorted callback methods.
  */
 
-typedef int (*DataDumperPtr) (Archive *AH, void *userArg);
+typedef int (*DataDumperPtr) (Archive *AH, const void *userArg);
 
 typedef void (*SetupWorkerPtrType) (Archive *AH);
 

src/bin/pg_dump/pg_backup_archiver.h

@@ -382,7 +382,7 @@ struct _tocEntry
 	int			nDeps;			/* number of dependencies */
 
 	DataDumperPtr dataDumper;	/* Routine to dump data for object */
-	void	   *dataDumperArg;	/* Arg for above routine */
+	const void *dataDumperArg;	/* Arg for above routine */
 	void	   *formatData;		/* TOC Entry data specific to file format */
 
 	/* working state while dumping/restoring */
@@ -421,7 +421,7 @@ typedef struct _archiveOpts
 	const DumpId *deps;
 	int			nDeps;
 	DataDumperPtr dumpFn;
-	void	   *dumpArg;
+	const void *dumpArg;
 } ArchiveOpts;
 #define ARCHIVE_OPTS(...) &(ArchiveOpts){__VA_ARGS__}
 /* Called to add a TOC entry */