Add schema file for gitlab-shell values

Towards #1852 (closed)

Closes: #1863 (closed)

Development decisions

1. Use one big schema file initially
   1. Error messages are not optimal when using individual validation files for subcharts
   2. It is relatively easy to split a global validation file to smaller ones in the future, if we need to. Each subchart's section is essentially a valid sub-schema, so we just need to take it out.
2. Not validating global parameters in the first iteration. Those default values will be taken from the root values.yaml file. We don't expect users to normally delete this file.
3. Initially validating only those values which are absolutely required to spin up an individual pod of the service. The criteria for success is essentially no error/warning logs in the pod.
4. Functionality tests are not done in the first iteration. "No error logs" is the only criterion we have now.

---

The following config got me a gitlab-shell pod that wasn't spitting out errors in logs (after deleting the gitlab-shell's values.yaml file)

global:
  gitaly:
    enabled: false
    external: ['default']
  grafana:
    enabled: false
  ingress:
    configureCertmanager: false
  minio:
    enabled: false

postgresql:
  install: false

prometheus:
  install: false

gitlab-runner:
  install: false

redis:
  install: false

registry:
  enabled: false

gitlab:
  gitlab-exporter:
    enabled: false
  migrations:
    enabled: false
  unicorn:
    enabled: false
  sidekiq:
    enabled: false
  task-runner:
    enabled: false
  gitlab-shell:
    config:
      loginGraceTime: 1
      maxStartups:
        full: 100
    deployment:
      livenessProbe: {}
    enabled: true
    hpa:
      targetAverageValue: 100m
    image:
      repository: registry.gitlab.com/gitlab-org/build/cng/gitlab-shell
    maxReplicas: 10
    service:
      name: gitlab-shell
      type: ClusterIP
      internalPort: 2222
    workhorse: {}