Add direct async iteration on Dataset and KeyValueStore

[vdusek]

Motivation

Crawlee JS recently added async iterator support (PR #3352) allowing direct iteration over storages:

for await (const item of dataset) { ... }
for await (const [key, value] of kvs) { ... }

Python has iterate_items() on Dataset and iterate_keys() on KeyValueStore, but lacks:

1. __aiter__ protocol on Dataset and KeyValueStore — no async for item in dataset syntax.
2. iterate_values() / iterate_entries() on KeyValueStore — can only iterate keys, not key-value pairs.

Proposed changes

Dataset

• Add __aiter__ delegating to iterate_items() so async for item in dataset works.

KeyValueStore

• Add iterate_values() — yields values.
• Add iterate_entries() — yields (key, value) tuples.
• Add __aiter__ delegating to iterate_entries() so async for key, value in kvs works.

JS reference

• PR: feat: implements async iterators crawlee#3352
• Dataset: packages/core/src/storages/dataset.ts
• KeyValueStore: packages/core/src/storages/key_value_store.ts