Re-export prost::Message

So that `Storable::encode_to_vec()` can be used without requiring
users to depend on the exact same prost version as vss-client.

Author: andrei-21

src/util/storable_builder.rs

@@ -1,5 +1,5 @@
 use crate::types::{EncryptionMetadata, PlaintextBlob, Storable};
-use ::prost::Message;
+pub use ::prost::Message;
 
 use chacha20_poly1305::{ChaCha20Poly1305, Key, Nonce};