http/python: Add simple HTTP GET example with FastAPI and Uvicorn

[ianmcook]

Closes apache/arrow#40869.

Supercedes #27

[ianmcook]

Note for posterity:

To adapt this to work with just plain Starlette (no FastAPI):

1. Replace the FastAPI imports with:

   from starlette.applications import Starlette
   from starlette.responses import StreamingResponse
   from starlette.routing import Route

2. Replace app = FastAPI() and everything below that with:

   def main(request):
       return StreamingResponse(
           generate_bytes(schema, batches),
           media_type="application/vnd.apache.arrow.stream"
       )
   
   app = Starlette(routes=[Route("/", endpoint=main)])

[ianmcook]

Tested and confirmed works with various client examples.