From what I can see, only SFTP processors use SSH keys so the function is a bit limited. However, the key and password and host_key properties must be entered for each SFTP processor. We also do not have a validator available to check if the SSH key file appears valid or not.
We could create an SSH Key service that contains these things and add a validator for some of the properties. The sshj library KeyProviderUtil may assist with checking if keys are valid files.
Relates to NIFI-3890