This commit adds support for supplying a path to an existing private
key file to both the `flux create secret git` and `flux create source
git` commands.
If a path is given, any private key generation configuration options
are ignored by the manifest generator. The SSH host will however still
be scanned for server keys.
Signed-off-by: Hidde Beydals <hello@hidde.co>
4750d0d81c