Backend: Implement `spec:inputs:` prerequisite for CI components

Context

Important read: https://gitlab.com/gitlab-org/gitlab/-/blob/master/doc/architecture/blueprints/ci_pipeline_components/index.md#input-parameters-specinputs-parameters

Previous discussions #377327 (closed) and !105817 (merged)

Proposal

As a first deliverable to start the implementation of CI components, we would like to allow the include: syntax to support input parameters. In the architecture blueprint we decided that a YAML configuration containing inputs: syntax requires the YAML including it to indicate input parameters using include:*:with syntax.

This proposal does not introduce any new component but leverages the existing capabilities our users use today using include:.

# .gitlab-ci.yml
include:
  - local: file.yml # support all include (local, project, remote, template, trigger: include, trigger: project)
    with:
      website: http://example.com

# file.yml to be included
spec:
  inputs:
    website: # required input. By default, all inputs are required.
    stage:
      default: test # optional input. 

---
    
# rest of the configuration content.
job1:
  stage: $[[ inputs.stage ]] # inputs interpolation.
  script: "./do-something $[[ inputs.website ]]"

job2:
  stage: $[[ inputs.stage ]]
  script: ./do-something-else

irb(main):002:0> YAML.load_stream(File.read('file.yml'))
=> [{"inputs"=>{"website"=>nil, "stage"=>{"default"=>"test"}}}, {"job1"=>{"stage"=>"$[[ inputs.stage ]]", "script"=>"./do-something $[[ inputs.website ]]"}, "job2"=>{"stage"=>"$[[ inputs.stage ]]", "script"=>"./do-something-else"}}]

spec:
  inputs:
    variable: KEY
---
rspec:
  script: echo Hello World

The MVC will consist of:

• Supporting spec:inputs:
  • Mandatory inputs
  • Default inputs
• Support using with: on all existing includes: (local, project, remote, template, trigger: include, trigger: project)
  • when optional inputs are provided they override the default value.
• Support inputs interpolation via $[[ inputs.XXX ]] syntax. Inputs are interpolated immediately after reading the file to include and before merging the YAML with the main configuration.

Validation

If someone uses with: but the including YAML doesn't specify spec:inputs: that should fail:

With:	Inputs:	Result
✔		error ❗
✔	✔	validate inputs
	✔	use default:
		Legacy include without inputs:

Out of scope (for consideration)

• Variable expansion on inputs:.
• inputs: on the main .gitlab-ci.yml (we will not support nor should we raise an error on inputs: as top level keyword in the main pipeline).
• Pre-fill Run Pipeline form with inputs (like pre-fill variables).
• Multiple inputs (options:).
• Input type validation.

Implementation Table

Title	MR Link	Done?
Add parser for multi-document YAML files	!109137 (merged)	✅
Update External::File::Base to accept multi-document YAML	!111221 (closed)
Add interpolation to CI configurations	!107208 (merged)	✅
Add Entry for spec:inputs: (including defaults)
Add Entry for with: (including validations)
Document new keywords