Noisy logging when redirections is not defined

[achamayou]

On a service without an interface called "primary_rpc_interface" and without a "redirections" anyway, this gets logged:

2024-08-16T11:10:55.912539Z -0.001 0   [fail ] ../src/node/rpc/frontend.h:378       | Could not find startup config for interface primary_rpc_interface

At a glance, it looks like we look up from context which interface a request came from, and find the relevant redirections mapping in the config. But when there is no interface, likely because the request was forwarded, we default to "primary_rpc_interface", which may or may not be there.

It's probably sensible to adopt a default redirection mapping, having a magic name is definitely not the way to do it.