Allow connections between routers

[Yadunund]

Fix the default router config to listen on all connections established to the 7447 port.

Will add documentation on how to connect two routers, as part of the fix for #102

[clalancette]

I'm hesitant about this one, from a security perspective. That is, with the current configuration, we are sure that nothing outside the current machine can possibly connect. If we enable this by default, then by default anyone can connect and gain access to the box.

Now, this situation isn't much different from DDS. But it still feels a little scary to do this by default. @codebot do you have any additional thoughts here?

[codebot]

I agree this opens up a plethora of security considerations, but I'm OK with it:

• this is the default behavior of ROS 1 (accept inbound connections on any interface)
• this is the default behavior of most other rmw_* in ROS 2, so I don't think people will be surprised.
• this situation is (IMO) considerably better than the default DDS behavior of multicasting discovery packets; although our default config would open up a port for inbound connections, at least it's not also sending multicast packets around the LAN and automatically connecting to peers.

In general, I think that people expect that deploying ROS 2 into an untrusted / hostile environment requires lots of thinking about lots of network-configuration and security things, layered defenses, key management, etc.

I think the higher priority for rmw_zenoh is a "just works" out-of-the-box experience, including "I want to use rviz on my laptop to see data on my robot without having to learn about middleware config files".

[codebot]

LGTM; this causes the default behavior to align with ROS 1 and most other rmw_* in ROS 2.