[tcgc] revisit tcgc method and operation design

many languages have concern about the relationship of method and operation in tcgc. also, we have concern about how we should honor (only see raw typespec or include some of the protocol info) for tcgc method level signature. and, current tcgc's method has inconsistency of parameters and responses. parameters are convenience level, but responses are protocol level. we should at lease either consider to make it consistent or switch to a middle line that emitter is easy to use. let's use this issue to start discussion.