plain/text responses are not support for OperationAsync

[fonzmeister]

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Describe the bug
The function ReadResourceFromMessage throws an exception whenever the response type is not a json/xml. However, for the $convert-data endpoint supported by fhir an plain/text is returned. Please refer to https://github.com/microsoft/fhir-server/blob/main/docs/ConvertDataOperation.md

Note the Content-Type is the response header is set to text/plain because the output format is determined by the mapping definition from your templates.

To Reproduce
Steps to reproduce the behavior:

1. send a $convert-data request using FhirClient.OperationAsync
2. See error:

Expected behavior
I would expect these return types to be supported since it is part of the fhir standard

Screenshots
If applicable, add screenshots to help explain your problem.

Version used:

• FHIR Version: r4
• Version: 5.9.0