Allow the verification of signatures and events

Added the methods:
- Event#verify_signature
- Crypto#check_sig!
- Crypto#valid_sig?
- Crypto#sign_message

Fixed a primitive obsession by introducing a Signature class to ensure that signatures are valid Nostr signatures.

sig/vendor/schnorr/signature.rbs

@@ -0,0 +1,16 @@
+# Added only to satisfy the Steep requirements. Not 100% reliable.
+module Schnorr
+  class InvalidSignatureError < StandardError
+  end
+
+  class Signature
+    attr_reader r: Integer
+    attr_reader s: Integer
+
+    def self.decode: (String string) -> Signature
+
+    def initialize: (Integer r, Integer s) -> void
+    def encode: -> String
+    def ==: (untyped other) -> bool
+  end
+end