Make pipectl the real cli console for PipeCD

[khanhtc1202]

The current pipectl has several commands (ref: docs) that support the use of PipeCD, but that's all, users are required to use PipeCD web console to finish many other tasks (especially around deployment stub, there is nothing to do from the pipectl cli console).

It's all right for the current situation due to PipeCD web console is friendly enough and good for the job, but some PipeCD potential users (aka. CD operators who love terminal console) may think it's better to have the terminal as the second console for PipeCD.

I think we can make it done without any blocks since:

• The design of PipeCD treats web/pipectl as the client (same from the server datastore point of view)
• The codebase for pipectl has no conflict with the other parts of the project

So I think it's time for making a cool/real terminal console for PipeCD 😎