feat: implement a basic backend (#1)

Reviewed-on: https://git.reya.su/reya/signed/pulls/1

crates/signed_nostr/src/lib.rs

@@ -0,0 +1,7 @@
+mod backend;
+mod signer;
+mod update;
+
+pub use backend::NostrBackend;
+pub use signer::{SignedAuthUrlHandler, UniversalSigner};
+pub use update::Update;