Set GITLAB_QA_USER_AGENT as a Rails env var

What does this MR do and why?

Sets GITLAB_QA_USER_AGENT as a Rails env var so that it can be used to bypass features that are allowlisted by GitLab itself (as opposed to external services, as in https://gitlab.com/gitlab-com/gl-infra/cloudflare-firewall/-/issues/63).

Also:

• Masks the credentials when they're shown as part of the omnibus-gitlab config settings.
• Allows other secrets to be masked via the mask_secrets keyword arg.

See https://gitlab.com/gitlab-org/quality/quality-engineering/team-tasks/-/issues/1599#note_1240112223

How to set up and validate locally

1. Set the env var

   export GITLAB_QA_USER_AGENT=<from 1Password>

2. Run the tests

   bundle exec exe/gitlab-qa Test::Instance::Image EE 

3. Note that the secret is masked:

   [Jan 17 2023 10:52:00 AEDT (Gitlab QA)] INFO  -- Shell command: `docker exec gitlab-ee-4390e310 bash -c "echo \"# Generated by GitLab QA 
   Omnibus Configurator at 2023-01-17 10:51:56
   # Gitlab::QA::Runtime::OmnibusConfigurations::Default
   gitlab_rails['gitlab_default_theme'] = 10 # Light Red Theme
   gitlab_rails['gitlab_disable_animations'] = true # Disable animations
   gitlab_rails['application_settings_cache_seconds'] = 0 # Settings cache expiry
   gitlab_rails['initial_root_password'] = '5iveL!fe' # Initial root password
   gitlab_rails['env'] = {'GITLAB_QA_USER_AGENT'=>'*****'}\" > /etc/gitlab/gitlab.rb;"`

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.