SFTP hook to prefer the SSH paramiko key over the key file path

[30blay]

Related issue: #16323

The SFTP hook does not allow for a SSH private key string to be used, only a private key file path. As it is based on the SSH hook, which does allow for a private key string to be used, a small change could align these two hooks.

This change first checks if a paramiko object is present after the SSH hook has been initialised, and the prefers that rather than the ssh key file path.

This PR replaces stale PR #16338

[boring-cyborg]

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[uranusjr]

Could you address the review comment in the original PR? Namely https://github.com/apache/airflow/pull/16338/files#r649058789

[30blay]

Could you address the review comment in the original PR? Namely https://github.com/apache/airflow/pull/16338/files#r649058789

Done!

[uranusjr]

Assuming CI passes.

[github-actions]

The PR is likely OK to be merged with just subset of tests for default Python and Database versions without running the full matrix of tests, because it does not modify the core of Airflow. If the committers decide that the full tests matrix is needed, they will add the label 'full tests needed'. Then you should rebase to the latest main or amend the last commit of the PR, and push it with --force-with-lease.

[boring-cyborg]

Awesome work, congrats on your first merged pull request!