[Feqture request] Add a way to produce server service implementation with using Stream directly

I'd love to use `grpclib.server.Stream` directly instead of `request_iterator/yield` combination to handle stream/steram gRPC server.

For example, the current way is a bit tricky to write consumer/producer style while `yield` must be placed directly under the function scope. So I need to use exception or some other tricky way to find if gRPC is closed or not.

```python
import asyncio
from contextlib import suppress

from .proto import ServerBase

# Assume that the followings are inbound/outbound for proxing gRPC messages
inbound: asyncio.Queue[str] = asyncio.Queue()
outbound: asyncio.Queue[str] = asyncio.Queue()

class ServerWithIteratorAndGenerator(ServerBase):
async def proxy(self, request_iterator: AsyncIterator[str]) -> AsyncIterator[str]:
class ConsumerClosedError(Exception):
pass

async def consumer_handler() -> None:
async for message in request_iterator:
await outbound.put(message)
raise ConsumerClosedError()

consumer = asyncio.create_task(consumer_handler())

with suppress(ConsumerClosedError):
while True:
producer = inbound.get()
done, _ = await asyncio.wait(
[consumer, producer],
return_when=asyncio.FIRST_COMPLETED,
)
# NOTE:
# If 'consumer' is closed (gRPC has closed), the code below raise ConsumerClosedError
message = await next(done)
if not message:
# inbound is closed.
break
yield message

# Make sure that consumer is closed
with suppress(ConsumerClosedError):
consumer.cancel()
```

If betterproto produce `ServerBase` with native `grpclib.server.Stream`, I could write above code as

```python
import asyncio

from .proto import ServerBase

# Assume that the followings are inbound/outbound for proxing gRPC messages
inbound: asyncio.Queue[str] = asyncio.Queue()
outbound: asyncio.Queue[str] = asyncio.Queue()

class ServerWithNativeStream(ServerBase):
async def proxy(self, stream: grpclib.server.Stream) -> None:
async def consumer_handler() -> None:
while True:
message = await stream.recv_message()
if message is None:
# gRPC is closed
break
await outbound.put(message)

async def producer_handler() -> None:
while True:
message = await inbound.get()
if message is None:
# inbound is closed
break
await stream.send_message(message)

consumer = asyncio.create_task(consumer_handler())
producer = asyncio.create_task(producer_handler())

done, pending = await asyncio.wait(
[consumer, producer],
return_when=asyncio.FIRST_COMPLETED,
)
for task in pending:
task.cancel()
for task in done:
task.result()
```

So above code does not have any tricky tech. to find if incoming stream is closed.

I know that I can use `_SeverBase__rpc_proxy()` method to overwrite it but I feel it's a bit redundant and ugly.

```python
import asyncio

from .proto import ServerBase

# Assume that the followings are inbound/outbound for proxing gRPC messages
inbound: asyncio.Queue[str] = asyncio.Queue()
outbound: asyncio.Queue[str] = asyncio.Queue()

class Server(ServerBase):
async def _ServerBase__rpc_proxy(self, stream: grpclib.server.Stream) -> None:
async def consumer_handler() -> None:
while True:
message = await stream.recv_message()
if message is None:
# gRPC is closed
break
await outbound.put(message)

async def producer_handler() -> None:
while True:
message = await inbound.get()
if message is None:
# inbound is closed
break
await stream.send_message(message)

consumer = asyncio.create_task(consumer_handler())
producer = asyncio.create_task(producer_handler())

done, pending = await asyncio.wait(
[consumer, producer],
return_when=asyncio.FIRST_COMPLETED,
)
for task in pending:
task.cancel()
for task in done:
task.result()
```