Add ARM helpers to CloudConfiguration

While not feasible for data-plane crates, ARM endpoints/authorities are very few. We should add helpers in `azure_core` per the comment thread: https://github.com/Azure/typespec-rust/pull/936#discussion_r3139723905

We'd then update the emitter to use those instead.

Read the linked comment thread first to identify the agreed ARM helpers, then inspect the azure_core CloudConfiguration area and the emitter entry point. Add the helpers there and update the emitter to use them instead; done means the ARM endpoint and authority handling follows that discussion and the relevant checks pass.

Written by the indexing model from the issue text.