Allow supplying PK from file for Git source/secret

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>