Add a message bus for transactional outbox deliveries

A service that updates database entities, and also writes a related
event to kafka can have some issues if we decide to wrap the database
updates in a transaction, then some error or delay occurs when writing
and event to kafka.  Especially if the event is written synchronously.

This can also cause lock conntention and increased resource useage
on the database server at scale.

* https://microservices.io/patterns/data/transactional-outbox.html
* https://docs.aws.amazon.com/prescriptive-guidance/latest/cloud-design-patterns/transactional-outbox.html

One solution is to write the event stream to a dedicated database
table, and have an additonal process to handle writing the events
to kafka.

This means that the application doesn't need to manage it's own
connections to kafka, and transactions can be used in the normal
way without any downsides or performance degredation.

This change provides a new OutboxMessageBus that can be configured with
an active record model.

e.g.

migration:
```
class CreateKafkaOutboxEvents < ActiveRecord::Migration[7.0]
  def change
    create_table :kafka_outbox_events do |t|
      t.string :topic
      t.string :key
      t.column :payload, :longblob # for avro - text would be more
appropriate for JSON
      t.timestamps
    end

    add_index :kafka_outbox_events, :topic
  end
end
```

config/initializers/streamy.rb:
```
require "streamy/message_buses/outbox_message_bus"
class KafkaOutboxEvent < ActiveRecord::Base; end
Streamy.message_bus = Streamy::MessageBuses::OutboxMessageBus.new(model: KafkaOutboxEvent)
```

This implimentation only allows for the use of a single table as the
outbox.  If we wanted to e.g. use a table per topic then the
implimentation will need to be a bit more complex.

For now, I suspect that indexing on the topic collum will
be good enough, as we can run multiple consuming workers
each selecting a different topic concurrently.

We will only be able to use a single worker to select rows (with
locking) in the consuming process where the backend is MySQL 5.7
however with an upgrade to MySQL 8+ we can make use of `SKIP LOCKED`
to increase concurrency if required.

lib/streamy/message_buses/outbox_message_bus.rb

@@ -0,0 +1,22 @@
+require "streamy/kafka_configuration"
+require "waterdrop"
+require "active_support/core_ext/hash/indifferent_access"
+require "active_support/json"
+
+module Streamy
+  module MessageBuses
+    class OutboxMessageBus < MessageBus
+      def initialize(config)
+        @model = config[:model]
+      end
+
+      def deliver(key:, topic:, payload:, priority:)
+        @model.create(key: key, topic: topic, payload: payload)
+      end
+
+      def deliver_many(messages)
+        @model.create(messages.map { |message| message.except(:priority) })
+      end
+    end
+  end
+end

test/message_buses/outbox_message_bus_test.rb

@@ -0,0 +1,101 @@
+require "test_helper"
+require "waterdrop"
+require "streamy/message_buses/outbox_message_bus"
+
+module Streamy
+  class OutboxMessageBusTest < Minitest::Test
+    attr_reader :bus
+
+    def setup
+      @model = mock("outbox_model")
+      @bus = MessageBuses::OutboxMessageBus.new(model: @model)
+    end
+
+    def example_delivery(priority)
+      bus.deliver(
+        payload: payload.to_s,
+        key: "prk-sg-001",
+        topic: "charcuterie",
+        priority: priority
+      )
+    end
+
+    def payload
+      {
+        type: "sausage",
+        body: { meat: "pork", herbs: "sage" },
+        event_time: "2018"
+      }
+    end
+
+    def expected_event(key: "prk-sg-001")
+      {
+        payload: {
+          type: "sausage",
+          body: {
+            meat: "pork",
+            herbs: "sage"
+          },
+          event_time: "2018"
+        }.to_s,
+        key: key,
+        topic: "charcuterie"
+      }
+    end
+
+    def test_standard_priority_deliver
+      @model.expects(:create).with(expected_event)
+      example_delivery(:standard)
+    end
+
+    def test_low_priority_deliver
+      @model.expects(:create).with(expected_event)
+      example_delivery(:low)
+    end
+
+    def test_essential_priority_deliver
+      @model.expects(:create).with(expected_event)
+      example_delivery(:essential)
+    end
+
+    def test_all_priority_delivery
+      @model.expects(:create).with(expected_event)
+      example_delivery(:essential)
+
+      @model.expects(:create).with(expected_event)
+      example_delivery(:low)
+
+      @model.expects(:create).with(expected_event)
+      example_delivery(:standard)
+    end
+
+    def test_batch_delivery
+      @model.expects(:create).with([
+        expected_event(key: "prk-sg-001"),
+        expected_event(key: "prk-sg-002"),
+        expected_event(key: "prk-sg-003")
+      ])
+
+      bus.deliver_many([
+        {
+          payload: payload.to_s,
+          key: "prk-sg-001",
+          topic: "charcuterie",
+          priority: :standard
+        },
+        {
+          payload: payload.to_s,
+          key: "prk-sg-002",
+          topic: "charcuterie",
+          priority: :standard
+        },
+        {
+          payload: payload.to_s,
+          key: "prk-sg-003",
+          topic: "charcuterie",
+          priority: :standard
+        }
+      ])
+    end
+  end
+end