HSS: Finish most of our configuration work
Here we make the right things serde, and apply the redesign that @Diziet and I talked about for the hsrproxy design. This implements a lot of #699 (closed).
Note that this MR disables the existing configuration option for authentication; I plan to get back to those with the design in #1028, but there's a lot of complexity there and I think we ought to figure out #1037 (closed) and #1038 (closed) first.