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>

apidoc/ostree-sections.txt

@@ -719,6 +719,7 @@ ostree_sign_get_by_name
 ostree_sign_get_name
 ostree_sign_detached_metadata_append
 ostree_sign_metadata_verify
+ostree_sign_add_pk
 ostree_sign_load_pk
 ostree_sign_set_pk
 ostree_sign_set_sk