[feat] `with` syntax for open_dataset


The with statement helps to ensure that the file is closed.  [8. Compound statements — Python 3.14.0 documentation](https://docs.python.org/3/reference/compound_stmts.html#the-with-statement)

This could be useful feature. 

```python
with metop_reader.open_dataset(file_path=str(test_file_path)) as ds:
    ## read some data

# metop_reader.close_dataset is automatically called.
```