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

doc/src/sgml/contrib.sgml

@@ -99,6 +99,7 @@ CREATE EXTENSION <replaceable>module_name</replaceable>;
  &amcheck;
  &auth-delay;
  &auto-explain;
+ &basic-archive;
  &bloom;
  &btree-gin;
  &btree-gist;