Avoid regexes when routing in `restJson1` and `rpcv2Cbor`

[david-perez]

Our router implementations for the restJson1 (see request_spec.rs) and rpcv2Cbor protocols rely on regexes.

We should consider code-generating a nom parser instead, which should be more performant. We already use nom parsers in restJson1 to extract @httpLabels from the URI (see #938), and some simple benchmarking found it to be ~200x faster ~200x faster.

It'd be interesting to tackle this after we have proper benchmarks (#214) to determine any performance impact.