Refactor PBS modules to go via the mux

[ltitanb]

Commit boost has a PBS module that can be extended/replaced (example). Given that there can be at most one PBS module per validator, it requires significant effort for module developers to get buy-in from node operators, and it's likely that many would want to experiment with only a subset of keys initially. Now that we have the PBS mux (and considering that relays implement the Builder API just like commit boost does), we should consider allowing external PBS modules to go via the mux only, allowing NOs to experiment more easily. Effectively this means the PBS module would mux to both relays (standard MEV boost behaviour), and to other PBS modules. The new flow becomes: CL -> commit boost PBS module -> extended PBS module -> mux relays, while without an additional sidecar (so eg muxing on just relays): CL -> commit boost PBS module -> mux relays.

With this we would be able to remove the builder API indirection (routes vs mev_boost). We should also modify the init command to make it easier to spin up a mux with a given sidecar (ie adding a docker image field to the mux config)

[ltitanb]

This would also solve issues like #105