Add support for user-defined functions and call field in YAML format

[garunitule]

implements #429

Examples:

functions:
  - name: my_function
    params: param1 param2
    command: python main.py $param1 $param2

steps:
  - name: step 1
    call:
      function: my_function
      args:
        param1: 1
        param2: 2

Notes

• If the general idea is good, I will implement the test

[garunitule]

Hi, @yohamta

May I ask you to run CI's and code reviews?

[yohamta]

LGTM, thanks for the amazing work! 👍

[garunitule]

Thank you very much!