plugin app manager: add a way to create files: a `files` field can now be used to create files on the fly. Its value must be a dict where key are file names, and values are an other dict with file data. For now file data can only use the `content` key. Value of content will be written to the file. If a file already exists, it is not overwritten.

version: "3.6"
services:

  backend:
    image: libervia/backend:dev
    build: backend-dev

  web:
    image: libervia/web:dev
    build: libervia-web-dev
    depends_on:
      - backend