Allow archiving via loadable modules.

Running a shell command for each file to be archived has a lot of
overhead and may not offer as much error checking as you want, or the
exact semantics that you want. So, offer the option to call a loadable
module for each file to be archived, rather than running a shell command.

Also, add a 'basic_archive' contrib module as an example implementation
that archives to a local directory.

Nathan Bossart, with a little bit of kibitzing by me.

Discussion: http://postgr.es/m/20220202224433.GA1036711@nathanxps13

contrib/basic_archive/expected/basic_archive.out

@@ -0,0 +1,29 @@
+CREATE TABLE test (a INT);
+SELECT 1 FROM pg_switch_wal();
+ ?column? 
+----------
+        1
+(1 row)
+
+DO $$
+DECLARE
+	archived bool;
+	loops int := 0;
+BEGIN
+	LOOP
+		archived := count(*) > 0 FROM pg_ls_dir('.', false, false) a
+			WHERE a ~ '^[0-9A-F]{24}$';
+		IF archived OR loops > 120 * 10 THEN EXIT; END IF;
+		PERFORM pg_sleep(0.1);
+		loops := loops + 1;
+	END LOOP;
+END
+$$;
+SELECT count(*) > 0 FROM pg_ls_dir('.', false, false) a
+	WHERE a ~ '^[0-9A-F]{24}$';
+ ?column? 
+----------
+ t
+(1 row)
+
+DROP TABLE test;