Add a `--listen-addr` flag to `dora coordinator` command

[XxChang]

add an optional --listen-port flags to dora coordinator command. If user sets this flag, coordinator will open an external control port as same as inner control port. However, every ControlRequest will be convert to ExternalControlEvent different than ControlEvent .

Separate external ControlRequest from ControlEvent for security.

[phil-opp]

Thanks for the PR! Could you explain the motivation behind this feature in some more detail? It looks like the start command is not allowed on the new port, otherwise it seems to behave the same. What is the security advantage of this?

As a general note, I don't think that a mix of "local" and "external" commands will be common. Instead, I expect the following two setups:

• Local deployment, either everything on same machine or in same local network.
  • We can probably consider all machines as trusted
• Remote deployment
  • The coordinator runs on a separate (cloud) instance
  • In this case, all coordinator requests are probably external (since the CLI is invoked on a different machine)

[XxChang]

In order to reuse the ControlRequest::Start in remote machine, Coordinator should know where the local_working_dir are located in, either external or inner working_dir. I haven't figured out how to handle external working_dir, so I leave it alone.

That is we can reuse the same ControlRequest in two different manners according to the source of request.

In addition, dora-cli should work in both remote device and local device (cloud instance). Identify where the dora-cli request comes from is useful to filter out unauthorized request.

All these are the future features, I'm leaving that blank now, so it behave the same currently but not future.