feat(pipelines): add archived field to Pipeline

Adds Archived bool to the Pipeline struct. GitLab returns archived on every pipeline response: get, latest, create, retry, cancel and update metadata. An archived pipeline can no longer be retried, so callers need the flag to tell it apart from a live one.

GitLab API docs: https://docs.gitlab.com/api/pipelines/#get-a-single-pipeline

Testing

TestGetPipeline_Archived checks the flag is decoded for both true and false. make reviewable passes.

Closes #2324 (closed)