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

[yohamta]

Add support for defining user-defined functions and a call field in the YAML format to enable more modular and reusable code within the DAGs.

Introduce a new top-level functions field for defining user-defined functions, and a new call field within the steps section to call those functions.

Example of defining a function and calling a function:

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

[garunitule]

@yohamta
May I work on it?

[yohamta]

Hi @garunitule 👋
That would be a huge help. Thank you so much!

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