Add MQTT Entity Sink Feature: Flat Topic Streaming feat: #535

[virajdere]

Adds feature #535

This pull request addresses #535 and introduces the MQTT Entity Sink feature to the cppagent, enabling observations to MQTT brokers using a flat, per-data-item topic structure.

Key changes include:

• New MqttEntitySink class and integration with agent configuration.
• Support for all relevant MQTT connection, authentication, and topic options.
• Documentation and configuration examples for agent setup.

Checklist

• Existing sinks unchanged
• All tests passing/No failures
• Builds cleanly
• Added additional tests for the feature
• Existing features remain unchanged/unaffected

Future development directions:

• Identify and fix any performance bottlenecks for the Entity format sink (currently no performance issues identified for brokers emqx/moquette as a part of initial testing)
• Add support for persistent buffer for all the data during broker disconnections/restarts.
• All persisted data should be relayed once network is restored
• Add support for durableRelay which allows users to configure whether they want to relay the persisted data
• This feature will be improved further with additional asset and device publishing

@wsobel