[tcgc] getWireName should honor content type

[tadelesh]

xml support in typespec have @encodedName to rename the xml tags name, however current tcgc does not honor content type when getWireName, but hard code to application/json. we need to fix that first. but an operation may have multiple content types, thus will have different wire name for different content type, since tcgc currently does not support multiple content type, we may do that when we add multi-content type support.