Analyze the Exists pattern

We decided to throw for 404s in MGC for HEAD requests, which is a departure from the Autorest behavior. The goal is that we will have some sort of [Azure decorator](https://github.com/Azure/typespec-azure/issues/1948) to handle this for Azure usage, but for unbranded, we think that provided the RequestOptions API is usable enough, users or library authors can add 404 as a success code if they want to avoid throwing. This issue can be closed with the result of the analysis and the decision for how this should be handled in MGC.