[Feature Request]: SolaceIO: Support user properties (message metadata) mapping #40099

What would you like to happen?

Problem statement

The Apache Beam Solace SDK currently drops the JCSMP user property map (XMLMessage.getProperties(), an SDTMap) in both directions:

• Read: SolaceRecordMapper.toRecord does not map getProperties() into Solace.Record, so all header-level metadata carried by the message is lost when reading from Solace.
• Write: SolaceRecordMapper.toMessage never calls setProperties(SDTMap), so it is impossible to publish user properties from a Beam pipeline.
  User properties are the recommended way to carry small header-level metadata fields alongside the payload. They are also the mechanism that interoperates across protocols: Solace translates the SDTMap user property map to/from MQTT 5 user properties and AMQP application properties. Supporting them is required for cross-protocol (MQTT 5 / AMQP / SMF) metadata interoperability.

Proposed change

Extend Solace.Record with a userProperties field of type Map<String, String>:

• Deserialization (toRecord): map each entry of the JCSMP SDTMap into the record, stringifying values (SDTField.toString()). A message without user properties maps to an empty map (never null), to avoid NPEs in application code.
• Serialization (toMessage): when the record carries a non-empty userProperties map, build an SDTMap via JCSMPFactory and set it with setProperties(...). When the map is empty, setProperties is not called, so published messages are byte-identical to the current behavior.
• The string-valued map is a deliberate, accepted trade-off aligned with MQTT 5, where user properties are strings. Users needing the original non-string types can cast back in their pipeline code. Typed SDT values are out of scope.

This continues the serialization/deserialization work introduced in #39875.

Issue Priority

Priority: 3 (nice-to-have improvement)

Issue Components

• Component: Python SDK
• Component: Java SDK
• Component: Go SDK
• Component: Typescript SDK
• Component: IO connector
• Component: Beam YAML
• Component: Beam examples
• Component: Beam playground
• Component: Beam katas
• Component: Website
• Component: Infrastructure
• Component: Spark Runner
• Component: Flink Runner
• Component: Prism Runner
• Component: Twister2 Runner
• Component: Hazelcast Jet Runner
• Component: Google Cloud Dataflow Runner