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.
1. The records are stored in redis which is not migratable to another cluster for the testing purposes.
2. Some of the requests fields are not recorded.
3. People should consider using https://github.com/buger/goreplay which is an existing mature and well-tested solution for testing purposes.
The function implementations of controller client package are
inconsistent. This PR lets functions reuse the functions
that already implemented and able to set additional headers to
request.
