Introduce Reports::Security::Remediation entity

This entity will provide us the interface to store remediation records
in database.

ee/lib/gitlab/ci/reports/security/remediation.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Ci
+    module Reports
+      module Security
+        class Remediation
+          attr_reader :summary, :diff
+
+          def initialize(summary, diff)
+            @summary = summary
+            @diff = diff
+          end
+
+          def diff_file
+            @diff_file ||= DiffFile.new(diff)
+          end
+
+          delegate :checksum, to: :diff_file
+
+          class DiffFile < StringIO
+            # This method is used by the `carrierwave` gem
+            def original_filename
+              "#{checksum}.diff"
+            end
+
+            def checksum
+              @checksum ||= Digest::SHA256.hexdigest(string)
+            end
+          end
+        end
+      end
+    end
+  end
+end

ee/spec/factories/ci/reports/security/remediations.rb

+# frozen_string_literal: true
+
+FactoryBot.define do
+  factory :ci_reports_security_remediation, class: '::Gitlab::Ci::Reports::Security::Remediation' do
+    summary { 'Remediation summary' }
+    diff { 'foo' }
+
+    skip_create
+
+    initialize_with do
+      ::Gitlab::Ci::Reports::Security::Remediation.new(summary, diff)
+    end
+  end
+end

ee/spec/lib/gitlab/ci/reports/security/remediation_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Ci::Reports::Security::Remediation do
+  let(:remediation) { build(:ci_reports_security_remediation) }
+
+  describe '#diff_file' do
+    subject { remediation.diff_file.read }
+
+    it { is_expected.to eq('foo') }
+  end
+
+  describe '#checksum' do
+    subject { remediation.checksum }
+
+    it { is_expected.to eq('2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae') }
+  end
+end