Make pipeline/job actions available through context menu

Problem to solve

Convenient for developer to operate GitLab Pipelines in VSCode.

Proposal

Add context menu to Pipeline sidebar.

Further details

Example: VSCode Sidebar

FOR CURRENT BRANCH
    |- Pipeline (Can run/retry/cancel pipeline and download all artifacts via context menu)
        |- Build (Can run/retry/cancel job, view job log and download job artifacts via context menu)
    |- Manual? (like `Run Pipeline` on Web UI)

Links / references