Load pipelines from local files

[lizardruss]

Is your feature request related to a problem?
Running shellcheck on pipeline code is currently not possible or cumbersome.

Which solution do you suggest?
Adding the ability to define pipeline code in external files would enable easier linting. An example might look like:

version: v2beta1
name: "pipeline-from-file"

pipelines:
  dev:
    script: dev.sh

This script would then be loaded from the local file system and run in the interpreted DevSpace shell environment.

Which alternative solutions exist?
A possible alternative would be to load the contents of a local file using cat, however this is not currently working. For example:

version: v2beta1
name: "pipeline-from-file"

pipelines:
  dev:
    run: $(cat dev.sh)

Additional context
Separate issue #2691 created for allowing files to be included via config expressions instead.