chore(russh-sftp): integrate russh-sftp as a vendor dependency

- Moved `russh-sftp` to the vendor directory and updated its path in `Cargo.toml`.
- Added new files for `russh-sftp`, including its `Cargo.toml`, `.gitignore`, and various source files for serialization, deserialization, and error handling.
- Implemented a basic structure for the SFTP client and server functionalities, enhancing the overall SSH file transfer capabilities.
- Introduced error handling and utility functions to support the new SFTP features.

src-tauri/Cargo.toml

@@ -96,7 +96,7 @@ tokio = { version = "1", features = [
     "process",
 ] }
 russh = { version = "0.62", features = ["des", "dsa"] }
-russh-sftp = { path = "crates/russh-sftp" }
+russh-sftp = { path = "vendor/russh-sftp" }
 ssh-key = { version = "0.6", features = ["std"] }
 portable-pty = "0.8"
 uuid = { version = "1", features = ["v4"] }