Extend JSON decoder field extraction and fix DomainMode propagation

[viacheslauK]

Summary

Extends what MQTTJSONDecoderFB can pull out of a JSON message: escaped dots and array indexes in paths, mixed integer/floating-point value arrays; fixes MqttSubscriberFbImpl silently dropping the Timestamp key when creating decoder FBs from a JSON config.

Changes

• Subscriber: setJsonConfig sets DomainMode explicitly. It was left at the default None, so readProperties discarded the DomainKey from the config and no domain signal was ever created for config-driven signals.
• Value types: an array's type is derived from all its elements rather than the first (detectArrayValueType). A mix of integers and doubles is promoted to Float64, an integers-only array stays Int64, any other mix reports an error. uint64 array elements are now read via GetUint64() and cast, matching the scalar path.
• Path syntax: resolveJsonPath supports array indexes (sensors[1].temperature, matrix[1][0]) and backslash escaping of ., [ and \ in field names.
• Docs and tests.

[NikolaiShipilov]

lgtm !