Set execution options in a way more consistent with DataFusion and DataFusion Python

Right now, the `DFRayContext` accepts options that govern its runtime behavior like
```python
ctx = DFRayContext(
batch_size=batch_size,
partitions_per_worker=partitions_per_worker,
prefetch_buffer_size=prefetch_buffer_size,
worker_pool_min=worker_pool_min,
```
While we cannot accept a `SessionConfig` from datafusion-python due to lack of ABI stability in rust, we should still try to adhere to the DataFusion Python API as much as possible.

So, we allow setting datafusion configuration options via `DFRayContext.set`.

Setting options in two places is confusion. I think it will be more clear if we set the options in the __init__ via `DFRayContext.set` instead.

We can call them
- `datafusion.ray.execution.batch_size`
- `datafusion.ray.execution.partitions_per_processor` (use updated name processor instead of worker here)
- `datafusion.ray.execution.prefetch_buffer_size`
- `datafusion.ray.execution.processor_pool_min` (use updated name processor)