Open Telemetry applied

Introduced Open Telemetry tracing together with context propagation:

• CQRS processing,
• Marten,
• EventStoreDB,
• Kafka.

Removed MediatR as:

• I'd like to have more customisation around Open Telemetry instrumentation,
• Retries,
• I haven't used either pipelines or behaviours,
• it was more limiting than helping (especially with its enforcement of marker interfaces).

Removed previous manual instrumentation around correlation and causation ids.

TODO:

• #184,
• #185.

See details in #181.