utils: Introduce || operator for named_values
utils: Introduce || operator for named_values

Those named_values that support .empty() check can be "selected"
like this

    auto& v = option_a() || option_b() || option_c();

This code will put into v a reference to the first non-empty
named_value out of a/b/c.

This "selection" is actually used on start when scylla decides
which config options to use as listen/broadcact/rpc/etc. addresses.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla