Improve SFTP hook's directory transfer to use a single connection for multiple files

[Dawnpool]

This PR improves SFTP hook's store_directory and retrieve_directory functions to use a single connection when transferring a directory with multiple files.

Previously, these functions relied on store_file and retrieve_file functions. And with this PR, the store_file and retrieve_file functions were modified to open and close sftp connection each time.
This leads to the store_directory and retrieve_directory functions to open and close too many connections repeatedly when there are many files in a directory, which causes significant overhead.

To address this, I modified them to open a connection, transfer all files in a directory, and then close the connection afterward.

I also did a performance test in my local environment by transferring a directory containing 1,000 small files. This reduced the transfer time by approximately 8-9 seconds. The results are shown below.

	AS-IS	TO-BE
store	47.18 sec	39.35 sec
retrieve	63.50 sec	54.73 sec