service-account commands #35

• service-account list | create | disable | delete
• service-account trust add | remove
• service-account key create | revoke

Depends on source-cooperative/source.coop#592 (service accounts, trust) and source-cooperative/source.coop#570 (keys).

Part of #21. Per the epic, the CLI validates nothing itself: it sends the request and shows the API's field_errors. Add a CLI path to docs.source.coop docs/using-source/ wherever this flow gets one.