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/nostr/errors/invalid_signature_format_error.rbs

@@ -0,0 +1,5 @@
+module Nostr
+  class InvalidSignatureFormatError < SignatureValidationError
+    def initialize: -> void
+  end
+end

sig/nostr/errors/invalid_signature_length_error.rbs

@@ -0,0 +1,5 @@
+module Nostr
+  class InvalidSignatureLengthError < SignatureValidationError
+    def initialize: -> void
+  end
+end

sig/nostr/errors/invalid_signature_type_error.rbs

@@ -0,0 +1,5 @@
+module Nostr
+  class InvalidSignatureTypeError < SignatureValidationError
+    def initialize: -> void
+  end
+end

sig/nostr/errors/signature_validation_error.rbs

@@ -0,0 +1,4 @@
+module Nostr
+  class SignatureValidationError < Error
+  end
+end