[Frontend]: Add variables deny/allow selector to pipeline execution policy

Why are we doing this work

As customers work to strike a balance between security/compliance and developer velocity, enforcement of CI jobs within project pipelines is only one dimension. There is also a desire to control the behavior for managing and manipulating variables for jobs in a project enforced by a pipeline execution policy.

Relevant links

Design

Non-functional requirements

• Documentation:
• Feature flag: security_policies_optional_variables_control
• Performance:
• Testing:

Implementation plan

1. Add new component according to design to ee/app/assets/javascripts/security_orchestration/components/policy_editor/pipeline_execution/action
2. Variables control for scan_execution policy in ee/app/assets/javascripts/security_orchestration/components/policy_editor/scan_execution/action/scan_filters/ci_variables_selectors.vue can be referenced as an example
3. Add test coverage
4. Can be broken down into multiple MR's or Issues

Verification steps