Streaming/SSE

TypeSpec needs to support streaming. In particular, there needs to be a way to:

- Declare that an operation takes and returns a conceptual stream
- A way to layer configuration of particular protocols on top, such as SSE
- Built-in support for common streaming protocols, e.g. JSONL and SSE
- Codegen support

```[tasklist]
### TypeSpec
- [ ] https://github.com/microsoft/typespec/issues/2378
```

```[tasklist]
### Spec
- [ ] https://github.com/microsoft/typespec/issues/6870
- [ ] https://github.com/Azure/typespec-azure/issues/613
- [ ] https://github.com/Azure/autorest.python/issues/2492
- [ ] https://github.com/Azure/autorest.java/issues/2672
- [ ] https://github.com/Azure/autorest.typescript/issues/2415
- [ ] https://github.com/Azure/azure-sdk-for-net/issues/47659
- [ ] https://github.com/Azure/autorest.cpp/issues/406
```

```[tasklist]
### Implementation
- [ ] https://github.com/Azure/typespec-azure/issues/614
- [ ] https://github.com/Azure/autorest.python/issues/2493
- [ ] https://github.com/Azure/autorest.java/issues/2673
- [ ] https://github.com/Azure/autorest.typescript/issues/2416
- [ ] https://github.com/Azure/azure-sdk-for-net/issues/47660
- [ ] https://github.com/Azure/autorest.go/issues/1450
- [ ] https://github.com/Azure/autorest.go/issues/1451
```