Add missing keywords to CI schema

What does this MR do and why?

For #218473 (closed)

This adds the missing keywords in our Gitlab CI schema. With this change, the pipeline editor will be able to lint the following rules:

• When to save the cache (cache:when)
• Trigger child pipeline with files from another project (trigger:includes)
• Rules in include

Screenshots or screen recordings

Rule	Recording	Notes
When to save the cache		Documentation
Trigger child pipeline with files from another project	trigger recording	Documentation. According to this discussion, file is required when project is given. ref is optional
Rules in include		Documentation. The docs mention that you can only use certain variables for include:rules:if but enforcing this is beyond the scope of the linter/json schema

How to set up and validate locally

1. Enable the :schema_linting feature flag. (rails c then Feature.enable(:schema_linting))

2. Visit to the pipeline editor by going to CI/CD > Editor

3. You can copy paste the following sample .gitlab-ci.yml content. The comments denote where the linter should complain about errors.

   There should also be a squiggly line underneath the keywords that have errors. You can hover on these to get more information on how to fix them. Fixing these errors should remove the linter complaints. You may also refer to the links above to read more on the rules the linter is enforcing.

include:
  - local: builds.yml
    rules: # this whole section should pass
      - if: '$INCLUDE_BUILDS == "true"'
        when: always

stages:
  - prepare

prepare_job:
  stage: prepare
  script:
    - echo 'running prepare_job'
  cache:
    when: 0 # error: should be a one of "on_success", "on_failure", "always"

child-pipeline:
  stage: prepare
  trigger:
    include:
      - project: 'my-group/my-pipeline-library' # error: missing 'file' property
      # should not complain if you add the optional `ref` property

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