Add a net::ToString() to help stringify ipaddr_t

Android, in particular, has problems with fmt's built-in variant
handling for this type for some reason.

llarp/net/interface_info.cpp

@@ -10,6 +10,6 @@ namespace llarp::net
         name,
         index,
         fmt::join(addrs, ","),
-        gateway ? fmt::format("{}", *gateway) : "none");
+        gateway ? net::ToString(*gateway) : "none");
   }
 }  // namespace llarp::net

llarp/net/net_int.cpp

@@ -1,6 +1,7 @@
 #include "net_int.hpp"
 #include "ip.hpp"
 #include <string>
+#include <variant>
 
 #include <oxenc/endian.h>
 
@@ -155,4 +156,11 @@ namespace llarp
   {
     return std::to_string(ntohs(n));
   }
+
+  std::string
+  net::ToString(const ipaddr_t& ipaddr)
+  {
+    return std::visit([](const auto& ip) { return ip.ToString(); }, ipaddr);
+  }
+
 }  // namespace llarp

llarp/net/net_int.hpp

@@ -230,6 +230,9 @@ namespace llarp
     using ipv6addr_t = n_uint128_t;
     using ipaddr_t = std::variant<ipv4addr_t, ipv6addr_t>;
 
+    std::string
+    ToString(const ipaddr_t& ip);
+
     huint16_t ToHost(port_t);
     huint32_t ToHost(ipv4addr_t);
     huint128_t ToHost(ipv6addr_t);