Backend: Implement multi-doc YAML parsing

Problem statement

Existing YAML parsing in GitLab is done by Gitlab::Config::Yaml::Loader. However, that class will only return a single document from a YAML file. We need to find a way to overcome the limitation in safely loading multi-document YAML so we can load both the spec and content from GitLab CI templates

Proposed solution

Implement suggestions in !109137 (merged)