This decouples Ci::Config to provide a common interface for handling
user configuration files.

Enables frozen string for the following:

* lib/gitlab/patch/**/*.rb
* lib/gitlab/popen/**/*.rb
* lib/gitlab/profiler/**/*.rb
* lib/gitlab/project_authorizations/**/*.rb
* lib/gitlab/prometheus/**/*.rb
* lib/gitlab/query_limiting/**/*.rb
* lib/gitlab/quick_actions/**/*.rb
* lib/gitlab/redis/**/*.rb
* lib/gitlab/request_profiler/**/*.rb
* lib/gitlab/search/**/*.rb
* lib/gitlab/sherlock/**/*.rb
* lib/gitlab/sidekiq_middleware/**/*.rb
* lib/gitlab/slash_commands/**/*.rb
* lib/gitlab/sql/**/*.rb
* lib/gitlab/template/**/*.rb
* lib/gitlab/testing/**/*.rb
* lib/gitlab/utils/**/*.rb
* lib/gitlab/webpack/**/*.rb

Partially addresses gitlab-org/gitlab-ce#47424.