Add support for custom MdcSerializer in EcsLayout

[aliaksandr-lavishak-tde]

Summary

EcsLayout from log4j2-ecs-layout currently uses a hardcoded implementation of MdcSerializer for MDC serialization. While this works well for most cases, it lacks flexibility for users with custom MDC serialization requirements.

I propose adding support for configuring or overriding the MdcSerializer implementation to accommodate specific use cases, such as:

• Custom JSON structure for MDC fields.
• Enhanced MDC sanitization or transformation logic.
• Integrating additional logging contexts.

Problem Statement

The current implementation of MdcSerializer is resolved dynamically using MdcSerializer.Resolver. However, it does not allow users to provide their own MdcSerializer implementation.

Proposed Solution

Introduce a mechanism to customize the MdcSerializer, such as already implemented for ObjectMessageJacksonSerializer

[aliaksandr-lavishak-tde]

I created an example implementation of how it could look.
#293