feat: add streaming download functionality for large files in AsyncFiles class

[kryachkow]

Applicable issues

#91

Description of changes

• Added true async HTTP streaming support in AsyncHTTPClient via a new stream(...) context manager that uses stream=True and closes responses safely.
• Added AsyncFiles.stream_download(...) to stream file downloads incrementally (without full buffering) while preserving existing error handling.
• Added README example for stream_download() usage on large async downloads.
• Added tests validating streamed chunk iteration, stream=True request behavior, response cleanup, and directory URL validation.

Checklist

• Title of the pull request follows Conventional Commits specification

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