* A Key struct, similar to tuf.Key, to be included in TUF metadata (not
yet implemented), which implements the DSSE Verifier interface, to
verify signatures created with Signer.
* A Signer struct, which implements the DSSE Signer interface, to create
signatures using `ssh-keygen` and a path to a key.
* An Import function, to import a Key using `ssh-keygen` and a path to a
key.
For signing and Key import paths to either public or private, plaintext
or encrypted, rsa, ecdsa or ed25519 keys are supported (akin to git's
user.signingKey configuration).
Also adds basic smoke tests for the `ssh` package, and replaces updates
dsse tests to use this module.
Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>
