feat: show CI pipelines and jobs

This MR introduces three types of side tree items: pipeline, stage, and job. As visible on the "after" screenshot:

before	after

The entry point for the whole feature is in current_branch_data_provider.ts where we replace the pre-exisitng logic to render a pipeline with the new PipelineItemModel. This model is responsible for showing pipeline and rendering StageItemModel which is then responsible for rendering the job items.

A detailed explanation of the VS Code API and what it allows us to display is in the related issue: #268 (comment 633175289)

The key part of the VS Code API to keep in mind is that our ItemModel implementations do two things, they return the item itself (getTreeItem() method) and they return children items (getChildren()) that can be either models or the items directly.

Related to #268 (closed)