Add `coverage_report` keyword to CI config

app/presenters/ci/build_runner_presenter.rb

doc/ci/yaml/artifacts_reports.md

...ser/project/merge_requests/test_coverage_visualization.md

lib/api/entities/ci/job_request/artifacts.rb

lib/gitlab/ci/config/entry/reports.rb

lib/gitlab/ci/config/entry/reports/coverage_report.rb

lib/gitlab/config/entry/validators.rb

..._ui/4_verify/pipeline/create_and_process_pipeline_spec.rb

spec/factories/ci/builds.rb

...ib/gitlab/ci/config/entry/reports/coverage_report_spec.rb

spec/lib/gitlab/ci/config/entry/reports_spec.rb

spec/lib/gitlab/config/entry/validators_spec.rb

spec/presenters/ci/build_runner_presenter_spec.rb

spec/requests/api/ci/runner/jobs_request_post_spec.rb

spec/services/ci/create_pipeline_service/artifacts_spec.rb