[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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[Suvrat1629]

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[Suvrat1629]

@robertwb @Abacn @svetakvsundhar Hey gentle ping. Could you please take a look and provide me with comments to improve this pr.
Thank you.

[Abacn]

Hi @svetakvsundhar would you mind taking a look of this HealthCareIO PR, thanks!

[svetakvsundhar]

Thank you for the contribution; 2 requests.

1. Could you please fix the spotless checks? The log is
   Error: eckstyle] [ERROR] /runner/_work/beam/beam/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/healthcare/HL7v2IO.java:101:68: Unclosed HTML tag found: <a [JavadocStyle]

2. Could you please add unit tests/integration tests to ensure that this is working end to end?

[Suvrat1629]

@svetakvsundhar I did fix the spotless checks and was working on the tests, I tried running the test file as it was and got the following error:org.apache.beam.sdk.util.UserCodeException: java.io.IOException: Your default credentials were not found. To set up Application Default Credentials for your environment. Is it necessary for me to make an account on google cloud or is there a way to test without creating a account?