[Feature Request] Support stack configs management

[zhaoyao91]

Thanks with a picture:

Now we could manage compose-file and env of a stack in a single place.

I recently encountered a case where I would like to deploy a nginx service which should be configured via config file. Thanks to configs feature, I could create a config and reference it in the stack compose file.

This works fine, but could be better if we can create files in the stack UI and reference it in compose file with config file syntax (see how they reference ./my_config.txt). In this way, we could group all related things in one place without the need to manage another global resource.