Ensure ConcatenationTable.cast uses target_schema metadata
Hi @lhoestq, Thanks for the detailed comment. I've tested the suggested approach and can confirm it works for the testcase outlined above! The PR is updated with the changes. _The documentation is not available anymore as the PR was closed or merged._
Currently, `ConcatenationTable.cast` does not use target_schema metadata when casting subtables. This causes an issue when using cast_column and the underlying table is a ConcatenationTable.

Code example of where issue arrises:
```
from datasets import Dataset, Image

column1 = [0, 1]
image_paths = ['/images/image1.jpg', '/images/image2.jpg']

ds = Dataset.from_dict({"column1": column1})
ds = ds.add_column("image", image_paths)
ds.cast_column("image", Image()) # Fails here
```
Output
```
...
TypeError: Couldn't cast array of type
string
to
{'bytes': Value(dtype='binary', id=None), 'path': Value(dtype='string', id=None)}
```
