refactor: Stop DeviceRef extending str

[tpoliaw]

This was originally to make serialization behave correctly when the task
was passed to requests. As we are using pydantic to dump the model to
JSON before passing it, we can add the custom serialization there to get
the device-to-string conversion that we need.

[codecov]

Codecov Report

❌ Patch coverage is 76.92308% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.60%. Comparing base (6d08154) to head (db9086c).

Files with missing lines	Patch %	Lines
src/blueapi/client/rest.py	40.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1557      +/-   ##
==========================================
- Coverage   95.69%   95.60%   -0.09%     
==========================================
  Files          43       43              
  Lines        3226     3231       +5     
==========================================
+ Hits         3087     3089       +2     
- Misses        139      142       +3     

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[oliwenmandiamond]

I did a quick test with this, seems to work

>>> devs.fsj1.
devs.fsj1.model  devs.fsj1.name  

No more unrelated methods. Tried some move plans.

>>> plans.set_absolute(devs.fsj1, "Out")
>>> plans.set_absolute(devs.fsj1, "In")
>>> plans.set_absolute(devs.smpm.x, 1)
>>> plans.set_absolute(devs.smpm.x, 5)

However, I think an unrelated change has broken the move plan again...

>>> plans.move({devs.fsj1, "Out"})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/scratch/bluesky_development/blueapi/src/blueapi/client/client.py", line 155, in __call__
    match self._client.run_task(req):
          ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/scratch/bluesky_development/blueapi/venv/lib64/python3.12/site-packages/observability_utils/tracing/decorators.py", line 151, in wrapper
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/scratch/bluesky_development/blueapi/src/blueapi/client/client.py", line 487, in run_task
    task_response = self._rest.create_task(task)
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/scratch/bluesky_development/blueapi/src/blueapi/client/rest.py", line 240, in create_task
    return self._request_and_deserialize(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/scratch/bluesky_development/blueapi/venv/lib64/python3.12/site-packages/observability_utils/tracing/decorators.py", line 151, in wrapper
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/scratch/bluesky_development/blueapi/src/blueapi/client/rest.py", line 311, in _request_and_deserialize
    response = self._pool.request(
               ^^^^^^^^^^^^^^^^^^^
  File "/scratch/bluesky_development/blueapi/venv/lib64/python3.12/site-packages/requests/sessions.py", line 575, in request
    prep = self.prepare_request(req)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/scratch/bluesky_development/blueapi/venv/lib64/python3.12/site-packages/requests/sessions.py", line 484, in prepare_request
    p.prepare(
  File "/scratch/bluesky_development/blueapi/venv/lib64/python3.12/site-packages/requests/models.py", line 370, in prepare
    self.prepare_body(data, files, json)
  File "/scratch/bluesky_development/blueapi/venv/lib64/python3.12/site-packages/requests/models.py", line 510, in prepare_body
    body = complexjson.dumps(json, allow_nan=False)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.12/json/__init__.py", line 238, in dumps
    **kw).encode(obj)
          ^^^^^^^^^^^
  File "/usr/lib64/python3.12/json/encoder.py", line 200, in encode
    chunks = self.iterencode(o, _one_shot=True)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.12/json/encoder.py", line 258, in iterencode
    return _iterencode(o, 0)
           ^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.12/json/encoder.py", line 180, in default
    raise TypeError(f'Object of type {o.__class__.__name__} '
TypeError: Object of type set is not JSON serializable

It resulted in error in main branch and this branch but I don't think it did yesterday... Something else could of broke it?