Added "Implementing Distributed Processes" sample

Added example of distributed processes management using Group Guests Checkout as an example.
It was modelled and explained in detail in the Implementing Distributed Processes Webinar:

It shows how to:

• orchestrate and coordinate business workflow spanning across multiple aggregates using Saga pattern,
• handle distributed processing both for asynchronous commands scheduling and events publishing,
• getting at-least-once delivery guarantee,
• implementing command store and outbox pattern on top of Marten and EventStoreDB,
• unit testing aggregates and Saga with a little help from Ogooreck,
• testing asynchronous code,

Read more in:

• Saga and Process Manager - distributed processes in practice
• Event-driven distributed processes by example.

See more in PR: #167.