Proxy logic for Boltz endpoint should depend on endpoint type

[tsjk]

Currently clboss uses a proxy to access a Boltz endpoint only if always-use-proxy is set in lightningd.conf.
It ought instead look at whether a clearnet address is available for the Boltz instance on the network it's on.
clboss uses a proxy not only for Boltz, but also for accessing DNS seeds, and hence the usage of proxy should be dependent on what is accessed.
We can currently work around the issue by defining a local address for instances that are not available on clearnet and requiring that the user dispatches an external proxy that listens on the predefined local address and forwards the traffic to the onion service via a tor client.