This PR adds a fake controller client for local CLI operations like
offline spec generation or for unit test purposes. The fake client
now only implements the "Version()" function and more functions
will be implemented once we figure out how to achieve the goals
mentioned above.
This PR adds an interface for controller API client, it allows us to
implement mock API client for unit testing with ease and we are
able to generate spec file without accessing the real Fission server.
Before this PR, CLI functions fatal out when encountering error
instead of returning it. Such behavior makes it hard to reuse
the functions nor writing unit tests. This PR aims to make functions
return errors instead of error out.
