Generalized variable substitution for LoopExecutor

[annaerdi]

Previously, the LoopExecutor only substituted variables in the cmd field. This was insufficient for commands with additional attributes, which may also contain placeholders.

This PR enhances the LoopExecutor to support generalized variable substitution for all string attributes in commands.

Units tests were written with the help of the sleep command to make sure it works with its seconds field. It was also successfully tested with the incoming browser executor. To manually test it with the sleep command, here is an example playbook:

vars:
  LISTA:
    - 1
    - 2
    - 3
    - 4
    - 5

commands:
  - type: loop
    cmd: "range(0,5)"
    commands:
      - type: sleep
        seconds: $LOOP_INDEX
  - type: loop
    cmd: "items(LISTA)"
    commands:
      - type: sleep
        seconds: $LOOP_ITEM