Add trigger support for matrix jobs

What does this MR do?

With this MR, downstream pipelines can be triggered with parallel:matrix keyword. And each downstream pipeline will receive respected variables assigned from matrix.

More info is in doc/ci/yaml/README.md change of this MR.

Related to #270957 (closed)

Screenshots (strongly suggested)

# .gitlab-ci.yml

test:
  stage: test
  script: echo test

deploy:
  stage: deploy
  trigger:
    include: child.yml
  parallel:
    matrix:
      - PROVIDER: ovh
        STACK: [monitoring, app]
      - PROVIDER: [gcp, vultr]
        STACK: [data]

# child.yml

test:
  script:
    - echo $PROVIDER
    - echo $STACK

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team