feat: add `pipelines` and `pipelines_extended` endpoints, update README

This MR has been created in course on my work to extend tap-gitlab. I realized that list pipelines and get single pipeline endpoints have different contents so also made the latter an optional (it's quite costly to query because every pipeline has to be a separate API call, but some teams are going to need it to track data like duration of a pipeline run etc).

I also adjusted gen_request() function to handle API requests that return not a list, but a single JSON object (like "get a single pipeline" endpoint).

Last but not least, I added released_at column to releases endpoint (must have been missed before?) and some README updates came as well.