feat(native): add SFTP client API

[lovewave02]

Summary

• add a native SFTPClient opened through SSHClient.open_sftp()
• expose cwd-aware directory, metadata, mutation, and transfer operations
• keep existing SSHClient SFTP methods compatible and document native-only support

Verification

• focused native SFTP unit and integration tests: 9 passed
• package unit tests: 29 passed
• flake8 pssh tests ci/integration_tests
• RST check and Sphinx HTML build
• existing native integration failures compared against origin/master: no new failures

Closes #410.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.43%. Comparing base (97a7414) to head (21807f0).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #426      +/-   ##
==========================================
- Coverage   99.53%   99.43%   -0.10%     
==========================================
  Files          18       19       +1     
  Lines        1703     1764      +61     
==========================================
+ Hits         1695     1754      +59     
- Misses          8       10       +2     

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[pkittenis]

Looks good as a first pass.

Would need to add a ParallelSFTPClient support for parallel operations, but for a single host SFTPClient this looks good.

Thank you for the PR, some minor changes needed for clarity and future parallel support.

[pkittenis]

Recursion would be nice to have here, but can be added later.

[pkittenis]

Would suggest just implementing a single host SFTPClient in this PR. The original PR was great, only changes needed were a rename of the client making function and returning a generator instead of list for the listdir implementation.

The parallel client I will take on as a separate PR once the single host client is merged.

[lovewave02]

Thanks for the guidance. I updated this PR to keep the single-host SFTPClient, renamed the helper to make_sftp_client, and made listdir generator-based. I also fixed the Python 3.8 SCP receive timeout; CircleCI is now green across Python 3.8-3.13. Could you take another look when convenient?