* API changes covered:
  * keys and interface modules removed
  * SSlibSigner removed
  * CryptoSigner added: this replaces the removed functionality
  * DSSE "signatures" container type changed
* Currently pins a securesystemslib main branch commit:
  this shoudl be reverted before merging, when securesystemslib
  has made a release
* tests/generated_data/generate_md.py was simplified
* Encrypted test keys in tests/repository_data/keystore were replaced
  with the unencrypted PEM versions of the same keys
* The public test keys in tests/repository_data/keystore were removed
  as they were not used anymore

Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>

private_bytes was just added to CryptoSigner, use it.

Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>

Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>