Move sentSzSave variable to state structure in wolfSSH_SFTP_SendWritePacket

This PR makes the following changes:

1. Moves the sentSzSave variable from being a local variable in the wolfSSH_SFTP_SendWritePacket function to being a member of the WS_SFTP_SEND_WRITE_STATE structure.
2. Adds a GitHub Action workflow to test SFTP file transfers with non-blocking sockets.

Changes Made

• Added sentSzSave as a member variable to the WS_SFTP_SEND_WRITE_STATE structure
• Removed the local variable declaration from the wolfSSH_SFTP_SendWritePacket function
• Added initialization of the new state member in the initialization state
• Updated all references to use state->sentSzSave instead of the local variable
• Created a GitHub Action workflow to test SFTP file transfers with non-blocking sockets

Testing

The changes have been tested locally using the following procedure:

1. Built wolfSSL with SSH support
2. Built wolfSSH with SFTP support
3. Started the echoserver in the background
4. Used wolfsftp with the -N flag (non-blocking sockets) to connect to the server
5. Used the put command to transfer a 2MB test file
6. Verified that the sent and received files matched exactly using MD5 checksums

The test confirmed that the changes did not affect the functionality of the SFTP file transfer and that the non-blocking mode works correctly.

Link to Devin run: https://app.devin.ai/sessions/ce8d743061084bff9e1dc755bd55c96e
Requested by: [email protected]