[tcgc] allow list: service and client definition

[tadelesh]

since we want to provide a good user experience for typespec author when using language's emitter, we want to use allow list mechanism to make it clear for user when writing some spec that emitter not supported. tcgc will try to identify whether a spec definition is allowed or not. if it is not allowed, some fallback logic will be done, and a diagnostic will be showed.
this issue is the first step. make sure all service and client definition story are clear. the task will be including service and client design doc for tcgc, usage doc for end user, tcgc validation, tcgc unit test, spector test.