# Functions
New create invocation, context can not be nil if you don't set ContextHeaderKey, then New will init it.
# Structs
Invocation is the basic struct that used in go chassis to make client and transport layer transparent .developer should implements a client which is able to transfer invocation to there own requesta protocol server should transfer request to invocation and then back to request.
Response is invocation response struct.
# Type aliases
ResponseCallBack process invocation response.