Introduce BridgeConfigBuilder

Fixes #604 (closed).

Apologies for the length of this branch. In particular, sorry that I didn't manage to break out the yaks into separate MRs, but they showed up quite late during preparation.

There's a fair amount of code here, basically doing BridgeConfibBuilder properly with a full deserialisation definition and a Rust API. As discussed in #604 (comment 2854966), I tried to avoid doing this now, but our existing config APIs really like these things to be done this way and after several attempts I couldn't get this to work in a way that wasn't very strange.