Implement variables for pipeline workflow rules [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This is another approach for #294232 (closed). First approach: !50682 (closed).

Refactor has been done before this MR => !57136 (merged)

This MR implements the variables keyword for rules of pipeline workflow. This is only for workflow:rules, the previous MR implemented for job:rules.

• Added root_variables to Pipeline::Chain::Command to store variables defined in YAML.
  • Initially, assigned result.workflow_attributes[:yaml_variables] in lib/gitlab/ci/pipeline/chain/config/process.rb.
• Added logic in lib/gitlab/ci/pipeline/chain/evaluate_workflow_rules.rb that applying rules for workflow by changing the root_variables of Pipeline::Chain::Command.
• We have logic in lib/gitlab/ci/config/entry/processable.rb that merging root variables with job variables.
  • Kept the variables attribute the same for now. It will be removed with #300581 (closed).
  • Added job_variables return only job variables.
  • Added variable_inheritance to handle this in seed/build.
• Then, added logic in lib/gitlab/ci/pipeline/seed/build.rb to override only variables from root.

Why did I choose this way?

• Our inherit logic is in lib/gitlab/ci/config/entry/processable.rb.
• However, we have the result of rules in the "create pipeline" logic and we need that result to override workflow variables.
• We need to use the variables from the result of rules to override the job variables from the workflow.
• We know the result of workflow:variables/workflow:rules:variables in seed/build.
• We need to merge root variables into job variables only if they are a part of "inheritance".
• Another comment: !52085 (comment 506395096)

---

• ci_workflow_rules_variables feature flag issue => #300997 (closed)

Screenshots (strongly suggested)

variables:
  VAR1: my var 1
  VAR2: my var 2

workflow:
  rules:
    - if: $CI_COMMIT_REF_NAME =~ /master/
      variables:
        VAR1: overridden var 1
    - if: $CI_COMMIT_REF_NAME =~ /feature/
      variables:
        VAR2: overridden var 2
        VAR3: new var 3
    - when: always

If the first condition is satisfied, then the total variables will be:

- VAR1: overridden var 1
- VAR2: my var 2

If the second condition is satisfied, then the total variables will be:

- VAR1: my var 1
- VAR2: overridden var 2
- VAR3: new var 3

If no condition is satisfied, then the total variables will be:

- VAR1: my var 1
- VAR2: my var 2

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