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.

Adds a rubocop rule (with autocorrect) to ensure line break after guard clauses.