Add rules support for CI pipeline include

What does this MR do?

This MR adds the support of include:rules:if.

• We don't support changes and exists for now because we don't have the pipeline object in the include phase.
• We only support project.predefined_variables, project.ci_instance_variables, group.ci_variables and project.ci_variables as variables. (https://gitlab.com/gitlab-org/gitlab/-/blob/3477d0fe692b4db08cef4cfd5e19dab63927315c/lib/gitlab/ci/config.rb#L111-133)
  • more info: #276515 (comment 636250569)

It's behind a feature flag ci_include_rules (#337507 (closed))

Related to #276515 (closed)

Screenshots or Screencasts (strongly suggested)

Normal run

Adding the variable

Run again

How to setup and validate locally (strongly suggested)

Example below:

1. Enable the feature flag

   Feature.enable(:ci_include_rules)

2. Use include:rules:if

   include:
     - local: builds.yml
       rules:
         - if: '$INCLUDE_BUILDS == "true"'
     - local: lints.yml
   
   test:
     stage: test
     script: exit 0

3. Run pipelines with and without having INCLUDE_BUILDS as a project variable.

Conformity

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Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
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Security

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• 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