refs #19589 - ensure that foreman starts in production

this avoids the following error in production (under passenger):
Message from application: uninitialized constant Generators::Base (NameError)
lib/generators/plugin/migration_generator.rb:2:in `<module:Plugin>'...

Fixes #19589 - Added plugin migration generator

Now plugin creators would be able to generate migrations by invoking

``` sh

rails g plugin:migration <migration name and options> --plugin_name=my_plugin

```