The dummy key manager impl from `arti-client` will be moved to
`tor-keymgr` soon. This commit adds a `keymgr` feature flag to
`tor-keymgr` which will eventually be used to choose between the real
key manager API and the dummy one.
This moves the filesystem calls from the `ssh` module to
`ArtiNativeKeyStore`. While `ArtiNativeKeyStore` shouldn't be concerning
itself with filesystem operations either, that refactoring will be
tackled separately (see arti#899).