[FE] Add ability to manage skip_ci setting in Policy UI editor for pipeline execution policy

Background

We're adding the ability to disable the [skip ci] restriction for Pipeline Execution Policies. This requires updating the frontend to allow users to configure this new option in the policy editor and view it in the policy drawer.

Objective

Update the Pipeline Execution Policy editor and drawer to include the new skip_ci configuration.

Implementation Details

Use ff security_policies_skip_ci
Policy Editor Updates:
- Add a new section for [skip ci] configuration
- Include a toggle switch for skip_ci.allowed
- Add a multi-select input for exceptions.users (visible when allowed is false)
- Implement user search functionality for the exceptions input
Form Submission:
- Include the new skip_ci configuration when saving or updating a policy
- Implement proper validation
UI/UX:
- Add tooltips and help text to explain the new options
- Ensure all new UI elements are accessible and follow GitLab's design system
Testing:
- Write unit and integration tests for the new UI components and logic

Acceptance Criteria

Users can configure skip_ci.allowed and exceptions in the policy editor
The policy drawer correctly displays the [skip ci] configuration
All new UI elements are responsive and follow GitLab's design guidelines
Error messages are displayed for invalid configurations
All new code is covered by appropriate tests

Design

yaml:

---
pipeline_execution_policy:
- name: My pipeline execution policy
  description: Enforces CI/CD jobs
  enabled: true
  pipeline_config_strategy: override_project_ci
  configuration:
    skip_ci:
      enabled: true
      allowed_for:
      - type: user
        id: 1231231
  content:
    include:
    - project: verify-issue-469027/policy-ci
      file: policy-ci.yml
      ref: main # optional
  policy_scope:
    projects:
      including:
      - id: 361