Draft: Introduce high level sftp file transfer api

Add a description of the new feature/bug fix. Reference any relevant bugs.

Checklist

• Commits have Signed-off-by: with name/author being identical to the commit author
• Code modified for feature
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTING.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code

Summary by Sourcery

Introduce a new high-level SFTP FT subsystem in libssh to simplify file transfers (upload, download, remote copy) with extended resume, metric tracking, and progress callback support, alongside a copy-range extension and buffer file I/O utilities, complete with tests, benchmarks, and documentation.

New Features:

• Introduce high-level SFTP file transfer API supporting upload, download, and remote copy with resume, metrics, and progress callbacks
• Implement sftp_copy_file_range to leverage the SFTP "copy-data" extension for direct file‐to‐file copying
• Add ssh_buffer_file_read and ssh_buffer_file_write helper APIs to load/store data between ssh_buffer and local files

Enhancements:

• Expose file transfer metrics (bytes transferred, total, skipped) via sftp_ft_get_* functions
• Support configurable chunk sizes and concurrent request counts in SFTP transfers
• Integrate SFTP FT API into existing benchmarks with new options for upload, download, and remote copy

Build:

• Update CMakeLists to compile and link new sftp_ft module and include corresponding tests and benchmarks

Documentation:

• Add user-facing documentation for the SFTP FT API in doc/sftp_ft.dox

Tests:

• Add comprehensive cmocka tests for the new SFTP FT API (including resume, chunk/request variations, and target file modes)
• Add torture tests for sftp_copy_file_range and new buffer read/write functions
• Extend unittests to cover ssh_buffer_file_read and ssh_buffer_file_write behaviors