Wire up the client side of the new `PATCH /client/system_settings`
endpoint added in zed-industries/cloud#2444, so we can persist the
currently selected organization on a per-system basis. This PR only adds
the request types and the client method; hooking it up to the actual
organization switcher in the editor will come next.

The endpoint requires the `x-zed-system-id` header (the server returns
400 without it), so the method does not take an Option like the other
client calls.

Release Notes:

- N/A