Draft: Implement reading inputs from various file sources in CI header specs

What does this MR do and why?

Implement local, remote and project/ref/file file includes for CI header specs

Add support for the include keyword in GitLab CI header specifications to read input definitions from local files. This enables sharing common input definitions across multiple configuration files.

Example usage:

---
spec:
  inputs:
    environment:
      default: production
  include:
    - local: '/ci/inputs.yml'
    - remote: 'http://gdk.test:3000/flightjs/ci-commit-ref-name/-/raw/test-external-file-inputs/ci/sample.yml'
    - project: 'flightjs/ci-commit-ref-name'
      file: 'ci/sample.yml'
      ref: 'test-external-file-inputs'
---

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Related to #415636