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.

spec/nostr/errors/invalid_signature_format_error_spec.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Nostr::InvalidSignatureFormatError do
+  describe '#initialize' do
+    let(:error) { described_class.new }
+
+    it 'builds a useful error message' do
+      expect(error.message).to eq('Only lowercase hexadecimal characters are allowed in signatures.')
+    end
+  end
+end

spec/nostr/errors/invalid_signature_length_error_spec.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Nostr::InvalidSignatureLengthError do
+  describe '#initialize' do
+    let(:error) { described_class.new }
+
+    it 'builds a useful error message' do
+      expect(error.message).to eq('Invalid signature length. It should have 128 characters.')
+    end
+  end
+end

spec/nostr/errors/invalid_signature_type_error_spec.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Nostr::InvalidSignatureTypeError do
+  describe '#initialize' do
+    let(:error) { described_class.new }
+
+    it 'builds a useful error message' do
+      expect(error.message).to eq('Invalid signature type')
+    end
+  end
+end