Add Perl interface to C storage layer.

Maintaining the storage layer/drivers in two languages is burdensome.  Since the integration tests require the Perl storage layer/drivers we'll need them even after the core code is migrated to C.  Create an interface layer so the Perl code can be removed and new storage drivers/features introduced without adding Perl equivalents.

The goal is to move the integration tests to C so this interface will eventually be removed.  That being the case, the interface was designed for maximum compatibility to ease the transition.  The result looks a bit hacky but we'll improve it as needed until it can be retired.

test/lib/pgBackRestTest/Module/Mock/MockAllTest.pm

@@ -878,7 +878,7 @@ sub run
 
             $oHostBackup->backup(
                 $strType, '$PGDATA is a substring of valid tblspc excluding / (file missing err expected)',
-                {oExpectedManifest => \%oManifest, iExpectedExitStatus => ERROR_FILE_MISSING});
+                {oExpectedManifest => \%oManifest, iExpectedExitStatus => ERROR_PATH_MISSING});
 
             testFileRemove("${strTblSpcPath}/99999");
         }