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>

src/libostree/libostree-devel.sym

@@ -33,6 +33,7 @@ global:
   ostree_sign_metadata_verify;
   ostree_sign_load_pk;
   ostree_sign_set_pk;
+  ostree_sign_add_pk;
   ostree_sign_set_sk;
   ostree_sign_dummy_get_type;
   ostree_sign_ed25519_get_type;