network: Only insert global addresses into the DHT.

Currently every address reported via libp2p-identify is inserted into
the DHT which thus contains a multitude of unreachable addresses such
as from 127.0.0.0/8 or 10.0.0.0/8.

Issue #5099 suggested a dedicated service over UDP to gauge the
reachability of an address, which would however incur extra I/O costs
and be of limited use.

As an alternative and simpler tactic, this PR only allows global IP
addresses to be inserted into the DHT unless an explicit command-line
flag `--allow-non-global-addresses-in-dht` is given or a node is
started with `--dev`. This opt-in behaviour is meant to allow
site-local networks to still make use of a DHT.