Define CI components syntax

Problem

We need to define a yaml syntax for the CI components. The component must define a name, a type (template, step, etc.), input parameters and optionally output parameters, as well as the component configuration/content.

Things to keep in consideration:

• In Allow pipeline name to be defined under workflow (!97502 - merged) we introduce workflow:name syntax for pipeline.
• We want to replace the design of pre-filled variables from using variables:description to an explicit list of pipeline inputs.
• We should move away from using variables to customize dynamic content and should instead use inputs.
• We could keep the UX and syntax as familiar as possible to existing use-cases so we can reuse functionalities.

Idea

# header/metadata: This part is stripped out when consuming the content (below).
workflow|template|step:
  name: My component X
  description: (Optional) My component X does Y very well
  inputs:
    environment:
      default: test
    website:
      required: true
    test_run:
      required: true
      options:
        - unit
        - integration
        - acceptance
        - system

# content
... # following the component type's schema

Input schema

inputs:
  deploy_environment:
    options: # render a selectbox with options in order of how they are defined below
      - staging    # 1st option
      - canary     # 2nd option
      - production # 3rd option
    default: staging # selected by default in the UI.
                     # if `default:` is not specified, the user must explicitly select
                     # an option.
    description: Deployment environment # render as input label.

An input can have:

• required: true (without default:) - in this case the input must be provided (value always expected).
• default: X - in this case it can be overridden or not provided and use the default (value always expected and required: true is implicit).
• none - in this case the input is optional.

Open discussion topics

• Is version: needed? Why? Ensure the blueprint is clear about that.
• Should component paths include CDN host gitlab.com?
• Where should a metadata file (applied to all components in a project) live? What about its syntax?
• Resolve discussion about directory structure and filename conventions.
• Ensure to resolve the discussion from !101762 (comment 1160546712)