net: relay I2P addresses even if not reachable (by us)

Nodes that can reach the I2P network (have set `-i2psam=`) will relay
I2P addresses even without this patch. However, nodes that can't reach
the I2P network will not. This was done as a precaution in
bitcoin/bitcoin#20119 before anybody could
connect to I2P because then, for sure, it would have been useless.

Now, however, we have I2P support and a bunch of I2P nodes, so get all
nodes on the network to relay I2P addresses to help with propagation,
similarly to what we do with Tor addresses.

Author: vasild

src/netaddress.h

@@ -227,7 +227,7 @@ class CNetAddr
          */
         bool IsRelayable() const
         {
-            return IsIPv4() || IsIPv6() || IsTor();
+            return IsIPv4() || IsIPv6() || IsTor() || IsI2P();
         }
 
         /**