Make PipelineCreaitonMetadata a proper model

app/graphql/types/ci/pipeline_creation.rb

app/graphql/types/ci/pipeline_creations/status_enum.rb

app/graphql/types/project_type.rb

app/models/ci/pipeline_creation/requests.rb

app/models/ci/pipeline_creation_metadata.rb

app/models/merge_request.rb

app/services/ci/create_pipeline_service.rb

app/services/merge_requests/create_pipeline_service.rb

app/workers/merge_requests/create_pipeline_worker.rb

...e_flags/gitlab_com_derisk/ci_redis_pipeline_creations.yml

doc/api/graphql/reference/index.md

ee/app/services/ee/merge_requests/create_pipeline_service.rb

...ervices/ee/merge_requests/create_pipeline_service_spec.rb

lib/api/merge_requests.rb

spec/graphql/types/ci/pipeline_creation_type_spec.rb

spec/graphql/types/ci/pipeline_creations/status_enum_spec.rb

spec/graphql/types/project_type_spec.rb

spec/models/ci/pipeline_creation/requests_spec.rb

spec/models/ci/pipeline_creation_metadata_spec.rb

spec/models/merge_request_spec.rb