OpenSSH-generated private keys do not work in App CR

When creating an App object using a ssh-git source and ssh private key, if the key was generated using openssh (default on MacOS as of this date) then it will fail complaining of key formatting.
```console
usefulErrorMessage: "Error: Syncing directory '0': Syncing directory '.' with git
contents: Fetching git repository: Git [fetch origin]: exit status 128 (stderr:
Failed to add the host to the list of known hosts (/home/kapp-controller/.ssh/known_hosts).\r\nLoad
key \"/etc/kappctrl-mem-tmp/kapp-controller-fetch-template-deploy420243064/.vendir-tmp/incoming/git-auth/private-key\":
invalid format\r\ngit@github.com: Permission denied (publickey).\r\nfatal: Could
not read from remote repository.\n\nPlease make sure you have the correct access
rights\nand the repository exists.\n)\n"
```

Start by tracing the ssh-git source handling used when creating an App object and reproduce the failure with an OpenSSH-generated private key. The issue names no source file or test; done means the key is accepted and the App sync completes without the invalid-format or publickey errors.

Written by the indexing model from the issue text.