sign: API changes for public keys and CLI keys format

sign: API changes for public keys and CLI keys format

API changes:
- added function `ostree_sign_add_pk()` for multiple public keys using.
- `ostree_sign_set_pk()` now substitutes all previously added keys.
- added function `ostree_sign_load_pk()` allowed to load keys from file.
- `ostree_sign_ed25519_load_pk()` able to load the raw keys list from file.
- use base64 encoded public and private ed25519 keys for CLI and keys file.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>