Add pull-policy support for service images on CI config

Steps

Step	Status
1. Runner: Implement allowed_pull_policies in config.toml	gitlab-runner!3422 (merged)
2. Runner: Implement supporting pull_policy from jobs	gitlab-runner!3412 (merged)
3. GitLab: Add pull-policy support for images	!85588 (merged)
4. Runner: Add image pull-policy support to services	gitlab-runner!3488 (closed)
5. GitLab: Add pull-policy support for service images on CI config	👈 You are here
6. Runner: Add the pull-policy from jobs support to Kubernetes	gitlab-runner!3504 (merged)

What does this MR do and why?

• This is the next step of #21619 (closed).
• The first iteration was to support pull_policy only for job images: !85588 (merged)

This MR implements the support of accepting pull_policy for services from CI config and sending it to Runner.

pull_policy can be either a string or an array of strings. More detail: https://docs.gitlab.com/runner/executors/docker.html#using-multiple-pull-policies

Notes:

• It's behind a feature flag ci_docker_image_pull_policy (#363186 (closed)).
• In order to make this work, gitlab-runner!3412 (merged) and gitlab-runner!3488 (closed) are required.
• We can merge this without the Runner work since this is behind a feature flag.

Screenshots or screen recordings

When you define a CI config;

job1:
  script: echo "this job uses postgres:11.7"
  services:
    - name: postgres:11.7

This is the default behavior:

---

Now you can define a pull_policy:

job2:
  script: echo "this job uses postgres:11.7"
  services:
    - name: postgres:11.7
      pull_policy: if-not-present

---

If you use the pull_policy "never" and there is no image in the Runner:

job3:
  script: echo "this job uses postgres:11.7"
  services:
    - name: postgres:11.7
      pull_policy: never

---

If the Runner does not allow the defined pull policy (allowed_pull_policies, gitlab-runner!3422 (merged))

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.