Modify UCObjectStore.list_objects to lists all files recursively

[irenedea]

What does this PR do?

Modifies UCObjectStore.list_objects to list all files recursively. Previously, UCObjectStore.list_objects only listed top level files.

This is consistent with other ObjectStore behavior.

Tests

(1) Manually tested with:

from composer.utils.object_store import UCObjectStore

uc = UCObjectStore(path="Volumes/main/nancyhung/cpt/small")
res = uc.list_objects(prefix="Volumes/main/nancyhung/cpt/small")
print('\n'.join(res))

Output:

/Volumes/main/nancyhung/cpt/small/AAA-fake/foo/chat.jsonl
/Volumes/main/nancyhung/cpt/small/ABT/sec_0000001800_10-K_1993_metadata.json
/Volumes/main/nancyhung/cpt/small/ABT/sec_0000001800_10-K_1993_txt.txt
/Volumes/main/nancyhung/cpt/small/ABT/sec_0000001800_10-K_1994_metadata.json
/Volumes/main/nancyhung/cpt/small/ABT/sec_0000001800_10-K_1994_txt.txt
...

I checked that each subfolder was represented.

Before these changes, there was no output printed because there were no top level files.

(2) Manually tested that an exception is raised when the max recursive depth is reached.

[panchalhp-db]

LGTM!