Added Marten Outbox Pattern/Subscription with custom Projection

[oskardudycz]

Added Marten Outbox Pattern/Subscription with custom Projection Plugged it into samples removing calling EventBus from the repository, by that getting at-least-once processing guarantee.

• MartenSubscription with general subscription logic,
• MartenEventPublisher to publish to in memory bus and forward to e.g. KafkaProducer or external database (e.g. ElasticSearch).

Read more about that in: https://event-driven.io/en/integrating_Marten/.

Other changes:

• Updated Marten to latest v5 alpha
• Disabled API tests parallelisation until better test setup is provided

See more in PR: #104

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