Generate postgres.gen.go using schema version directory structure (#808)

* Reorganize the db migration .sql scripts into individual schema version directories and generate the sql httpfs fakeFile accordingly.

* Created a /dev schema version directory to hold db schema updates that have not been finalized.

* Add an 'allow-development-migrations' flag to allow `boundary database init` to be executed even when there are schema updates in /dev.

internal/db/migrations/postgres/0/22_static_host.down.sql

@@ -0,0 +1,17 @@
+begin;
+
+  drop table static_host_set_member cascade;
+  drop table static_host_set cascade;
+  drop table static_host cascade;
+  drop table static_host_catalog cascade;
+
+  delete
+    from oplog_ticket
+   where name in (
+          'static_host_catalog',
+          'static_host',
+          'static_host_set',
+          'static_host_set_member'
+        );
+
+commit;