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.

[D-BUS Service]
Name=org.libervia.Libervia
Exec=libervia-backend