Allow users to configure skip ci behavior for pipeline execution policy

What does this MR do and why?

This MR adds ability to disable [skip ci] restriction for Pipeline Execution Policies. Users now will have option to allow skipping pipelines created from Pipeline Execution Policies with [skip ci] and to specify users that should be exempted from that restriction.

References

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• Make [ci skip] configurable for pipeline execut... (&15647)

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Before	After

How to set up and validate locally

1. Create new group and new project in that group
2. Create new Pipeline Execution Policy (Secure -> Policies -> Create new -> Pipeline Execution Policy) and merge MR with the policy
3. Go to project created in 1., modify the file and commit change with [skip ci] annotation. -> Pipeline will run
4. Now go back to Secure -> Policies and modify the policy created in 2.
5. Add skip_ci: { allowed: false, allowlist: { users: [{ id: YOUR_USER_ID }] }}
6. Go to project created in 1., modify the file and commit change with [skip ci] annotation. -> Pipeline will be skipped
7. Now go back to Secure -> Policies and modify the policy created in 2.
8. Add skip_ci: { allowed: true }
9. Go to project created in 1., modify the file and commit change with [skip ci] annotation. -> -> Pipeline will be skipped

---
pipeline_execution_policy:
- name: PEP with skip ci enabled
  description: ''
  enabled: true
  pipeline_config_strategy: inject_ci
  content:
    include:
    - project: root/pep-skip-ci
      file: pep.yml
  skip_ci:
    allowed: false
    allowlist:
      users:
      - id: 1
approval_policy: []

Related to [BE] Add ability to skip_ci to pipeline executi... (#507831 - closed)