Support allowed_hosts in global.appConfig

Summary

I notice there seems to be no way in Charts to set allowed_hosts (gitlab-org/gitlab!55491 (merged))

Ideally we have a way to set allowed_hosts to better protect GitLab in cloud deployments.

production:
  gitlab:
    allowed_hosts:
      - gitlab.example.com
      - gitlab.dr.example.com
      - gitlab.internal.dev

See documentation for Omnibus: https://docs.gitlab.com/omnibus/settings/configuration.html#set-allowed-hosts-to-prevent-host-header-attacks

Configuration used

(Please provide a sanitized version of the configuration used wrapped in a code block (```yaml))

global:
  appConfig:
    allowed_hosts:
      - gitlab.example.com
      - gitlab.dr.example.com
      - gitlab.internal.dev

Current behavior

Unable to configure production.gitlab.allowed_hosts[] in gitlab.yml of affected Rails deployments.

Expected behavior

Ability to configure production.gitlab.allowed_hosts[] in gitlab.yml of affected Rails deployments.