Use config mods to fill in fields in the config

[rcannood]

It might be useful if we could use config mods to query jsons inside of a config. For example:

functionality:
  name: foo
  arguments:
    - name: "--input"
      type: file
      default: ${.functionality.name}

(Not saying this particular example is useful, but there are other usecases where the functionality is useful)

Additionally, being able to use external files to fill in parts of a json would also be very useful. Example:

functionality:
  name: foo
  description: file://README.md}
  arguments:
    - inherit: ${file://api/unprocessed.yaml}
    - inherit: ${file://api/processed.yaml}
      direction: output

Before we start implementing something. let's first discuss whether we should and what the interface should look like.