feat: Support subtasks and mixins

[x1unix]

This PR adds support of calling sub-tasks and mixins, which are reusable pieces of pipeline.

Examples

Mixins

Mixins allow decoupling repeated pieces of pipeline into separate blocks.

Mixins, like tasks, support accepting inputs.

mixins:
  hello:
    inputs:
      name:
        type: string
      greeting:
        type: string
        default: Hello
    steps:
      - action: debug/echo
        with:
          message: "${{inputs.greeting}}, ${{inputs.name}}!"
      - action: debug/echo
        delay: 300ms
        with:
          message: "Goodbye, ${{inputs.name}}"

tasks:
  test:
    steps:
      - mixin: hello
        with:
          name: Alice 

Subtasks

Tasks can be called inside another tasks, similar to mixins:

tasks:
  foo:
    steps:
      ...
  bar:
    steps:
      - task: foo # foo calls bar