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

Enables frozen string for the following:

* lib/gitlab/ci/*.rb
* lib/gitlab/ci/build/**/*.rb
* lib/gitlab/ci/config/**/*.rb
* lib/gitlab/ci/pipeline/**/*.rb
* lib/gitlab/ci/reports/**/*.rb

Partially addresses #47424.

This whitelists all existing offenses for the various CodeReuse cops, of
which most are triggered by the CodeReuse/ActiveRecord cop.

This reverts commit e00fb2bdc2090e9cabeb1eb35a2672a882cc96e9.

# Conflicts:
#	.rubocop.yml
#	.rubocop_todo.yml
#	lib/gitlab/ci/config/entry/global.rb
#	lib/gitlab/ci/config/entry/jobs.rb
#	spec/lib/gitlab/ci/config/entry/factory_spec.rb
#	spec/lib/gitlab/ci/config/entry/global_spec.rb
#	spec/lib/gitlab/ci/config/entry/job_spec.rb
#	spec/lib/gitlab/ci/status/build/factory_spec.rb
#	spec/lib/gitlab/incoming_email_spec.rb

This reverts commit cb10b725c8929b8b4460f89c9d96c773af39ba6b.