[AIRFLOW-7044] Add ability to specify host_key in connections extras for SSH

[aaronfowles]

This PR adds a new option in the SSH connection extras where you can specify base64 ssh-rsa public key of a host. The SSHHook constructor will then add this key to ~/.ssh/known_hosts if not present.

NB - this is my first time submitting to this project.

Issue link: AIRFLOW-7044

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