Allow creation of a numpy array soft signal

[DominicOram]

When I try and do this I get an error of:

ERROR:blueapi.worker.reworker:invalid converter for value of type <class 'numpy.ndarray'>
Traceback (most recent call last):
  File "/venv/lib/python3.11/site-packages/blueapi/worker/reworker.py", line 250, in _cycle
    self._current.task.do_task(self._ctx)
  File "/venv/lib/python3.11/site-packages/blueapi/worker/task.py", line 31, in do_task
    ctx.run_engine(func(**prepared_params.dict()))
  File "/venv/lib/python3.11/site-packages/bluesky/run_engine.py", line 918, in __call__
    plan_return = self._resume_task(init_func=_build_task)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/bluesky/run_engine.py", line 1057, in _resume_task
    raise exc
  File "/venv/lib/python3.11/site-packages/bluesky/run_engine.py", line 1687, in _run
    raise err
  File "/venv/lib/python3.11/site-packages/bluesky/run_engine.py", line 1547, in _run
    msg = self._plan_stack[-1].send(resp)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/blueapi-plugins/scratch/mx_bluesky/src/mx_bluesky/i04/thawing_plan.py", line 95, in thaw_and_center
    yield from _thaw_and_center()
  File "/venv/lib/python3.11/site-packages/bluesky/utils/__init__.py", line 1203, in dec_inner
    return (yield from plan)
            ^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/bluesky/preprocessors.py", line 408, in subs_wrapper
    return (yield from finalize_wrapper(_inner_plan(),
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/bluesky/preprocessors.py", line 535, in finalize_wrapper
    ret = yield from plan
          ^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/bluesky/preprocessors.py", line 406, in _inner_plan
    return (yield from plan)
            ^^^^^^^^^^^^^^^
  File "/blueapi-plugins/scratch/mx_bluesky/src/mx_bluesky/i04/thawing_plan.py", line 87, in _thaw_and_center
    yield from bps.monitor(oav.test_signal, name="oav")
  File "/venv/lib/python3.11/site-packages/bluesky/plan_stubs.py", line 146, in monitor
    return (yield Msg('monitor', obj, name=name, **kwargs))
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/bluesky/run_engine.py", line 1607, in _run
    new_response = await coro(msg)
                   ^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/bluesky/run_engine.py", line 1950, in _monitor
    await current_run.monitor(msg)
  File "/venv/lib/python3.11/site-packages/bluesky/bundlers.py", line 392, in monitor
    await self._ensure_cached(obj)
  File "/venv/lib/python3.11/site-packages/bluesky/bundlers.py", line 207, in _ensure_cached
    await asyncio.gather(*coros)
  File "/venv/lib/python3.11/site-packages/bluesky/bundlers.py", line 337, in _cache_describe
    self._describe_cache[obj] = await maybe_await(obj.describe())
                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/bluesky/utils/__init__.py", line 1852, in maybe_await
    return await ret
           ^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/ophyd_async/core/signal.py", line 41, in wrapper
    return await asyncio.wait_for(func(self, *args, **kwargs), self._timeout)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/asyncio/tasks.py", line 489, in wait_for
    return fut.result()
           ^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/ophyd_async/core/signal.py", line 195, in describe
    return {self.name: await self._backend.get_datakey(self.source)}
                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/ophyd_async/core/soft_signal_backend.py", line 186, in get_datakey
    return self.converter.get_datakey(source, self._value, **self._metadata)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/ophyd_async/core/soft_signal_backend.py", line 60, in get_datakey
    dtype in primitive_dtypes
AssertionError: invalid converter for value of type <class 'numpy.ndarray'>

I can use a list to get round this but numpy arrays are valid as normal PV signal types and so should be valid here

[coretl]

Might be fixed by #522