Add NotifyService for generic alert notifications

Create a duplicated but simplified version of the
`Projects::Prometheus::Alerts::NotifyService`.
The service still uses a Prometheus payload structure.

These changes should help move towards generic alerts by modifying
a payload and extract alert presenters.

ee/app/services/projects/alerting/notify_service.rb

+# frozen_string_literal: true
+
+module Projects
+  module Alerting
+    class NotifyService < BaseService
+      include Gitlab::Utils::StrongMemoize
+
+      def execute
+        process_incident_issues if create_issue?
+
+        true
+      end
+
+      private
+
+      def generic_alert_endpoint_enabled?
+        Feature.enabled?(:generic_alert_endpoint, project)
+      end
+
+      def incident_management_available?
+        project.feature_available?(:incident_management)
+      end
+
+      def create_issue?
+        incident_management_available? && generic_alert_endpoint_enabled?
+      end
+
+      def process_incident_issues
+        IncidentManagement::ProcessAlertWorker
+          .perform_async(project.id, params.to_h)
+      end
+    end
+  end
+end

ee/spec/services/projects/alerting/notify_service_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Projects::Alerting::NotifyService do
+  set(:project) { create(:project) }
+
+  before do
+    # We use `set(:project)` so we make sure to clear caches
+    project.clear_memoization(:licensed_feature_available)
+  end
+
+  shared_examples 'processes incident issues' do |amount|
+    let(:create_incident_service) { spy }
+
+    it 'processes issues', :sidekiq do
+      expect(IncidentManagement::ProcessAlertWorker)
+        .to receive(:perform_async)
+        .with(project.id, kind_of(Hash))
+        .exactly(amount).times
+
+      Sidekiq::Testing.inline! do
+        expect(subject).to eq(true)
+      end
+    end
+  end
+
+  shared_examples 'does not process incident issues' do
+    it 'does not process issues' do
+      expect(IncidentManagement::ProcessAlertWorker)
+        .not_to receive(:perform_async)
+
+      expect(subject).to eq(true)
+    end
+  end
+
+  describe '#execute' do
+    subject { service.execute }
+
+    let(:starts_at) { Time.now.change(usec: 0) }
+    let(:service) { described_class.new(project, nil, payload) }
+    let(:payload_raw) do
+      {
+        'annotations' => {
+          'title' => 'annotation title'
+        },
+        'startsAt' => starts_at.rfc3339
+      }
+    end
+    let(:payload) { ActionController::Parameters.new(payload_raw).permit! }
+
+    context 'with license' do
+      before do
+        stub_licensed_features(incident_management: true)
+      end
+
+      context 'with Generic Alert Endpoint feature enabled' do
+        before do
+          stub_feature_flags(generic_alert_endpoint: true)
+        end
+
+        it_behaves_like 'processes incident issues', 1
+      end
+
+      context 'with Generic Alert Endpoint feature disabled' do
+        before do
+          stub_feature_flags(generic_alert_endpoint: false)
+        end
+
+        it_behaves_like 'does not process incident issues'
+      end
+    end
+
+    context 'without license' do
+      before do
+        stub_licensed_features(incident_management: false)
+      end
+
+      it_behaves_like 'does not process incident issues'
+    end
+  end
+end