MessageHubConsumer: add startPositionStr parameter to enable start position at submission time

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The enhancement for the Kafka toolkit covered by Kafka toolkit issue 213, should be made available also for the MessageHub toolkit.

• base this toolkit on streamsx.kafka 3.1.0
• provide a sample
• update documentation on https://ibmstreams.github.io/streamsx.messagehub/

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resolved in v3.2.0