Document how to handle non-serializable plan args

On i04 we have two plans that we want GDA to be able to call (`plan_a` and `plan_b`). `plan_a` is basically a super-set of `plan_b` except we want to inject a bit of extra logic into the middle of `plan_b`. I tried to implement this as:

```python

def plan_a() -> MsgGenerator:
...
def subplan():
...
yield from plan_b(subplan)

def plan_b(subplan:Callable[[], MsgGenerator]=None) -> MsgGenerator:
...
if subplan:
yield from subplan()
...
```

However, I go the error of:

```
Traceback (most recent call last):
File "/venv/lib/python3.11/site-packages/uvicorn/protocols/http/h11_impl.py", line 406, in run_asgi
result = await app( # type: ignore[func-returns-value]
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/venv/lib/python3.11/site-packages/uvicorn/middleware/proxy_headers.py", line 70, in __call__
return await self.app(scope, receive, send)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/venv/lib/python3.11/site-packages/fastapi/applications.py", line 1054, in __call__
await super().__call__(scope, receive, send)
File "/venv/lib/python3.11/site-packages/starlette/applications.py", line 113, in __call__
await self.middleware_stack(scope, receive, send)
File "/venv/lib/python3.11/site-packages/starlette/middleware/errors.py", line 187, in __call__
raise exc
File "/venv/lib/python3.11/site-packages/starlette/middleware/errors.py", line 165, in __call__
await self.app(scope, receive, _send)
File "/venv/lib/python3.11/site-packages/starlette/middleware/base.py", line 185, in __call__
with collapse_excgroups():
File "/usr/local/lib/python3.11/contextlib.py", line 158, in __exit__
self.gen.throw(typ, value, traceback)
File "/venv/lib/python3.11/site-packages/starlette/_utils.py", line 83, in collapse_excgroups
raise exc
File "/venv/lib/python3.11/site-packages/starlette/middleware/base.py", line 187, in __call__
response = await self.dispatch_func(request, call_next)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/venv/lib/python3.11/site-packages/blueapi/service/main.py", line 342, in add_api_version_header
response = await call_next(request)
^^^^^^^^^^^^^^^^^^^^^^^^
File "/venv/lib/python3.11/site-packages/starlette/middleware/base.py", line 163, in call_next
raise app_exc
File "/venv/lib/python3.11/site-packages/starlette/middleware/base.py", line 149, in coro
await self.app(scope, receive_or_disconnect, send_no_error)
File "/venv/lib/python3.11/site-packages/starlette/middleware/exceptions.py", line 62, in __call__
await wrap_app_handling_exceptions(self.app, conn)(scope, receive, send)
File "/venv/lib/python3.11/site-packages/starlette/_exception_handler.py", line 62, in wrapped_app
raise exc
File "/venv/lib/python3.11/site-packages/starlette/_exception_handler.py", line 51, in wrapped_app
await app(scope, receive, sender)
File "/venv/lib/python3.11/site-packages/starlette/routing.py", line 715, in __call__
await self.middleware_stack(scope, receive, send)
File "/venv/lib/python3.11/site-packages/starlette/routing.py", line 735, in app
await route.handle(scope, receive, send)
File "/venv/lib/python3.11/site-packages/starlette/routing.py", line 288, in handle
await self.app(scope, receive, send)
File "/venv/lib/python3.11/site-packages/starlette/routing.py", line 76, in app
await wrap_app_handling_exceptions(app, request)(scope, receive, send)
File "/venv/lib/python3.11/site-packages/starlette/_exception_handler.py", line 62, in wrapped_app
raise exc
File "/venv/lib/python3.11/site-packages/starlette/_exception_handler.py", line 51, in wrapped_app
await app(scope, receive, sender)
File "/venv/lib/python3.11/site-packages/starlette/routing.py", line 73, in app
response = await f(request)
^^^^^^^^^^^^^^^^
File "/venv/lib/python3.11/site-packages/fastapi/routing.py", line 301, in app
raw_response = await run_endpoint_function(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/venv/lib/python3.11/site-packages/fastapi/routing.py", line 214, in run_endpoint_function
return await run_in_threadpool(dependant.call, **values)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/venv/lib/python3.11/site-packages/starlette/concurrency.py", line 39, in run_in_threadpool
return await anyio.to_thread.run_sync(func, *args)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/venv/lib/python3.11/site-packages/anyio/to_thread.py", line 56, in run_sync
return await get_async_backend().run_sync_in_worker_thread(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/venv/lib/python3.11/site-packages/anyio/_backends/_asyncio.py", line 2177, in run_sync_in_worker_thread
return await future
^^^^^^^^^^^^
File "/venv/lib/python3.11/site-packages/anyio/_backends/_asyncio.py", line 859, in run
result = context.run(func, *args)
^^^^^^^^^^^^^^^^^^^^^^^^
File "/venv/lib/python3.11/site-packages/blueapi/service/main.py", line 110, in get_plans
return PlanResponse(plans=runner.run(interface.get_plans))
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/venv/lib/python3.11/site-packages/blueapi/service/runner.py", line 91, in run
return self._run_in_subprocess(function, *args, **kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/venv/lib/python3.11/site-packages/blueapi/service/runner.py", line 112, in _run_in_subprocess
return self._subprocess.apply(
^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.11/multiprocessing/pool.py", line 360, in apply
return self.apply_async(func, args, kwds).get()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.11/multiprocessing/pool.py", line 774, in get
raise self._value
pydantic.errors.PydanticInvalidForJsonSchema: Cannot generate a JsonSchema for core_schema.CallableSchema
```

It's understandable that a `Callable` cannot be used in a pydantic model and supplied by GDA but I never need to supply `subplan` from GDA. This can be solved by refactoring into a 3rd plan that the two both call but we could also have a general rule of if an arg is optional and not serialiasable then do not worry about exposing it? Probably with a message to the user