[Java] Add parsedData to Hl7v2Message and Update HL7v2IO Docs

[Suvrat1629]

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This PR addresses #20452 by adding support for the parsedData field in the Hl7v2Message class, enabling users of the HL7v2IO connector who do not rely on schematized parsing to utilize HL7v2 messages from the Google Cloud Healthcare API.

Fixes #20452

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