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.

lib/nostr/errors/invalid_signature_format_error.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+module Nostr
+  # Raised when the signature is in an invalid format
+  #
+  # @api public
+  #
+  class InvalidSignatureFormatError < SignatureValidationError
+    # Initializes the error
+    #
+    # @example
+    #   InvalidSignatureFormatError.new
+    #
+    def initialize
+      super('Only lowercase hexadecimal characters are allowed in signatures.')
+    end
+  end
+end

lib/nostr/errors/invalid_signature_length_error.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+module Nostr
+  # Raised when the signature's length is not 128 characters
+  #
+  # @api public
+  #
+  class InvalidSignatureLengthError < SignatureValidationError
+    # Initializes the error
+    #
+    # @example
+    #   InvalidSignatureLengthError.new
+    #
+    def initialize
+      super('Invalid signature length. It should have 128 characters.')
+    end
+  end
+end

lib/nostr/errors/invalid_signature_type_error.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+module Nostr
+  # Raised when the signature is not a string
+  #
+  # @api public
+  #
+  class InvalidSignatureTypeError < SignatureValidationError
+    # Initializes the error
+    #
+    # @example
+    #   InvalidSignatureTypeError.new
+    #
+    def initialize = super('Invalid signature type')
+  end
+end

lib/nostr/errors/signature_validation_error.rb

@@ -0,0 +1,6 @@
+# frozen_string_literal: true
+
+module Nostr
+  # Base class for all signature validation errors
+  class SignatureValidationError < Error; end
+end