Burr Serving Framework

**Is your feature request related to a problem? Please describe.**
We have some examples of serving in FastAPI. They all follow the same pattern. We should be able to automate this (to some extent):
- [streaming chatbot](https://blog.dagworks.io/p/streaming-chatbot-with-burr-fastapi)
- [email assistant](https://towardsdatascience.com/building-an-email-assistant-application-with-burr-324bc34c547d)

**Describe the solution you'd like**
A few thoughts:

In FastAPI:

```python
burr_app = Application()....build()
app = FastAPI()
burr_fastapi.post("/api/v0/...", app, burr_app, halt_after=..., halt_before=..., ..., response_model=CustomResponseModel)
burr_fastapi.post_streaming("/api/v0/...", app, burr_app, halt_after=..., halt_before=..., ..., response_model=CustomResponseModel)
```

Then `CustomResponseModel` has to be instantiatable from state dict (E.G. be a subset/have the right defaults).

Could also decorate functions -- these inject the results in

```python
burr_app = Application()....build()
app = FastAPI()
@burr_fastapi.post("/api/v0/...", app, burr_app, halt_after=..., halt_before=..., ...)
def my_burr_endpoint(burr_state, burr_action, burr_result) -> MyResponse:
return MyResponse(...)
```

Note that we could also derive it if we set up the fields we expose in state + add #139 -- this is something we'd want to do later, but...

```python
burr_app = ...
app = FastAPI()
burr_fastapi.post("/api/v0/...", app, burr_app, halt_after=..., halt_before=..., ...)
```

This could automatically derive the response model based on types statically inferred + halt_before/halt_after, although it's a little complicated...

**Describe alternatives you've considered**
Can do custom like we do now.

**Additional context**
As asked on discord -- this is part 1

Review the linked FastAPI serving examples and the current custom serving pattern before narrowing the scope. Compare the proposed burr_fastapi.post and post_streaming entry points, including halt controls and response-model/state behavior, and account for the relationship to follow-up #139. Done should mean an agreed first-part API with documented acceptance tests.

Written by the indexing model from the issue text.