List Slack invoices
List Slack billing statements from the workspace's admin billing history. Returns workspace, total and invoices (date, statement, billId, amount, status, description, pdfUrl) — metadata only. Feed a row's pdfUrl (or billId) to slack.com/download_invoice to fetch that statement's PDF. Requires a workspace billing admin; defaults to the primary workspace, or pass workspaceDomain (e.g. acme.slack.com).