[open_channel] open a private channel when pubic is none

[gpBlockchain]

https://github.com/nervosnetwork/fiber/blob/78a4da816e9dafb502ae8f992de4b770d58e045b/src/rpc/channel.rs#L405

[chenyukang]

@quake @contrun it's better to set default channel to be public?

[quake]

I think so, I will create a PR for this issue.