Support `worker_threads` option
The amount of threads for a worker is currently hardcoded to the amount of cores:
- https://gitlab.kwant-project.org/qt/dask-quantumtinkerer/-/blob/ca8787f5a0c6d5f9be3e23d295ede782425aaf3d/dask_quantumtinkerer/server.py#L233
- https://github.com/dask/dask-gateway/blob/0.9.0/dask-gateway-server/dask_gateway_server/backends/db_base.py#L1445
This function is useful for when you do not want multiple threads within a worker to interfere with each other. For example in this issue: https://github.com/dask/dask/issues/5229