Draft: Implement variables for pipeline workflow rules

What does this MR do?

This is just a draft. Just testing if the way is good or not. No need to review refactorings for now.

Related to #294232 (closed)

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 yaml_variables attr_accessor in Ci::Pipeline to store variables defined in YAML.
• Added logic in lib/gitlab/ci/pipeline/chain/evaluate_workflow_rules.rb that applying rules for workflow by changing the yaml_variables of Ci::Pipeline.
• We have logic in lib/gitlab/ci/config/entry/processable.rb that merging root variables with job variables. I've added the source attribute to this logic in order to determine where variables come from.
• Then, added logic in lib/gitlab/ci/pipeline/seed/build.rb to override only variables with the source workflow.

Why did I choose this way?

• Our inherit logic is in lib/gitlab/ci/config/entry/processable.rb, so it looks like that we need to apply "inheriting" variables in here.
• 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.

TODO: Add a feature flag

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: on_success

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